diff --git a/CLAUDE.md b/CLAUDE.md
index c752491..6e49ad9 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -30,10 +30,16 @@ A custom mastering toolkit that provides metrics to evaluate audio masterings th
 - Real-time analysis display and file management
 
 #### `analysis_results_manager.py`
-- Background threading for audio analysis
+- Background threading for audio analysis (`AudioAnalysisWorker` = load + first
+  metric; `MetricComputeWorker` = one metric on an already-loaded file)
 - Caches both the loaded `AudioFile` and per-metric `compute()` output, so
   metric/font switches re-render from cache without reloading librosa
-- Progress tracking and error handling
+- **Prefetch** (`PrefetchWorker`): after a file loads, the remaining metrics are
+  computed in the background (one at a time, cooperatively cancellable) so the
+  first switch to any metric is instant too. Superseded when a new file loads
+- Timing: workers measure compute time; `metricTiming` + phase/duration progress
+  messages drive the status slip ("X computed in Ys", "Loaded in Ns — computing…")
+- `shutdown()` stops all threads on window close (`MainWindow.closeEvent`)
 
 #### `audio_visualization_widget.py`
 - Persistent pyqtgraph plot — the PlotItem is reused across renders, never torn
@@ -82,10 +88,12 @@ A custom mastering toolkit that provides metrics to evaluate audio masterings th
 - Current registry:
   - `RMSPowerMetric` — 10 s rolling RMS with adaptive colour scale
   - `WaveformMetric` — min/max envelope, fixed ±1.1 y-range
-  - `LUFSMetric` — true (ungated) EBU R128 short-term (3 s) computed via a single
-    K-weighting pass (`_short_term_lufs`, reusing pyloudnorm's filter
-    coefficients) + a vectorised sliding mean-square; integrated + LRA still come
-    from pyloudnorm (one gated call each). ~2× faster than the old per-window loop
+  - `LUFSMetric` — true (ungated) EBU R128 short-term (3 s) via a single
+    K-weighting pass (`_kweight`, cached) + a vectorised sliding mean-square.
+    Integrated (`_integrated_lufs`) and LRA (`_loudness_range`) are reimplemented
+    from the same cached K-weighted signal — validated **bit-equal** to
+    pyloudnorm — so nothing re-filters the signal. ~3.8 s → ~0.6 s. pyloudnorm is
+    now used only to source the BS.1770 filter coefficients
   - `CrestFactorMetric` — 20·log10(peak/RMS) per 1 s window; peaks via O(N) running max
   - `PSRMetric` — sample-peak minus short-term LUFS (3 s window); reuses
     `LUFSMetric`'s short-term series (memoised on the `AudioFile`), so PSR is
@@ -211,7 +219,8 @@ A custom mastering toolkit that provides metrics to evaluate audio masterings th
 - numpy: Numerical computations
 - scipy: Signal processing (true-peak polyphase oversampling, K-weighting
   filters, spectrogram log-frequency resample, O(N) running-max via ndimage)
-- pyloudnorm: BS.1770 loudness (LUFS, LRA)
+- pyloudnorm: source of the BS.1770 K-weighting filter coefficients (the LUFS
+  short-term / integrated / LRA math is now computed directly, validated against it)
 - pyqtgraph: Interactive plotting (zoom/pan, overlay, lin/log)
 - matplotlib: Colormaps only (consumed by pyqtgraph) + librosa dependency
 - mutagen: Audio metadata extraction
diff --git a/analysis_results_manager.py b/analysis_results_manager.py
index b0154f5..76084e1 100644
--- a/analysis_results_manager.py
+++ b/analysis_results_manager.py
@@ -7,6 +7,7 @@ from PyQt5.QtCore import QObject, pyqtSignal, QThread
 from dataclasses import dataclass, field
 from typing import Any, Optional
 import os
+import time
 import logging
 
 from master_core import AudioFile
@@ -49,16 +50,21 @@ class AudioAnalysisWorker(QThread):
 
   def run(self):
     try:
-      self.logger.info(f"Starting analysis of: {os.path.basename(self.file_path)}")
-      self.progressUpdate.emit("Loading audio file...", 10)
+      base = os.path.basename(self.file_path)
+      self.logger.info(f"Starting analysis of: {base}")
 
+      # Decode is a black box (no progress callback), so report it as a phase
+      # with its measured duration rather than a fake percentage.
+      self.progressUpdate.emit(f"Loading {base}…", 0)
+      t0 = time.perf_counter()
       audio_file = AudioFile(self.file_path)
-      self.progressUpdate.emit("Audio loaded...", 30)
+      load_s = time.perf_counter() - t0
 
-      self.progressUpdate.emit(f"Computing {self.metric.display_name}...", 60)
+      self.progressUpdate.emit(
+        f"Loaded in {load_s:.1f}s — computing {self.metric.display_name}…", 50)
+      t1 = time.perf_counter()
       metric_data = {self.metric.id: self.metric.compute(audio_file)}
-
-      self.progressUpdate.emit("Finalizing analysis...", 90)
+      metric_s = time.perf_counter() - t1
 
       result = AnalysisResult(
         file_path=self.file_path,
@@ -70,8 +76,12 @@ class AudioAnalysisWorker(QThread):
         analysis_successful=True,
       )
 
-      self.progressUpdate.emit("Analysis complete!", 100)
-      self.logger.info(f"Analysis completed: {os.path.basename(self.file_path)}")
+      self.logger.info(
+        f"Analysis completed: {base} (load {load_s:.2f}s, "
+        f"{self.metric.id} {metric_s:.2f}s)")
+      self.progressUpdate.emit(
+        f"{self.metric.display_name} ready in {metric_s:.1f}s "
+        f"(loaded in {load_s:.1f}s)", 100)
       self.analysisCompleted.emit(self.file_path, result)
 
     except Exception as e:
@@ -83,8 +93,8 @@ class AudioAnalysisWorker(QThread):
 class MetricComputeWorker(QThread):
   """Worker thread that computes a single metric against an already-loaded AudioFile."""
 
-  completed = pyqtSignal(str, str, object)  # file_path, metric_id, data
-  failed = pyqtSignal(str, str, str)         # file_path, metric_id, error_message
+  completed = pyqtSignal(str, str, object, float)  # file_path, metric_id, data, seconds
+  failed = pyqtSignal(str, str, str)               # file_path, metric_id, error_message
 
   def __init__(self, file_path: str, audio_file: AudioFile, metric: Metric):
     super().__init__()
@@ -98,14 +108,53 @@ class MetricComputeWorker(QThread):
       self.logger.info(
         f"Computing {self.metric.display_name} for {os.path.basename(self.file_path)}"
       )
+      t0 = time.perf_counter()
       data = self.metric.compute(self.audio_file)
-      self.completed.emit(self.file_path, self.metric.id, data)
+      elapsed = time.perf_counter() - t0
+      self.completed.emit(self.file_path, self.metric.id, data, elapsed)
     except Exception as e:
       msg = f"{self.metric.display_name} compute failed: {e}"
       self.logger.error(msg)
       self.failed.emit(self.file_path, self.metric.id, str(e))
 
 
+class PrefetchWorker(QThread):
+  """Background worker that warms the cache by computing the remaining metrics.
+
+  Runs the given metrics sequentially on an already-loaded AudioFile so that
+  switching to any metric is instant the first time too. Cooperative: `stop()`
+  lets it bail between metrics (e.g. when a new file supersedes it). Skips any
+  metric that got computed on-demand in the meantime.
+  """
+
+  computedOne = pyqtSignal(str, str, object)  # file_path, metric_id, data
+
+  def __init__(self, file_path: str, result: "AnalysisResult", metrics: list):
+    super().__init__()
+    self.file_path = file_path
+    self.result = result
+    self.metrics = metrics
+    self._stop = False
+    self.logger = logging.getLogger(__name__)
+
+  def stop(self):
+    self._stop = True
+
+  def run(self):
+    for metric in self.metrics:
+      if self._stop:
+        return
+      if metric.id in self.result.metric_data:
+        continue  # already computed on-demand while we were working
+      try:
+        data = metric.compute(self.result.audio_file)
+        if self._stop:
+          return
+        self.computedOne.emit(self.file_path, metric.id, data)
+      except Exception as e:
+        self.logger.warning(f"Prefetch of {metric.id} failed: {e}")
+
+
 class AnalysisResultsManager(QObject):
   """Manages audio file analysis and coordinates between processing and GUI."""
 
@@ -119,12 +168,14 @@ class AnalysisResultsManager(QObject):
   metricComputeStarted = pyqtSignal(str, str)   # file_path, metric_id
   metricReady = pyqtSignal(str, str)            # file_path, metric_id
   metricComputeError = pyqtSignal(str, str, str)  # file_path, metric_id, error
+  metricTiming = pyqtSignal(str, str, float)    # file_path, metric_id, seconds
 
   def __init__(self):
     super().__init__()
     self.results_cache: dict[str, AnalysisResult] = {}
     self.current_worker: Optional[AudioAnalysisWorker] = None
     self.metric_workers: dict[tuple[str, str], MetricComputeWorker] = {}
+    self.prefetch_worker: Optional[PrefetchWorker] = None
     self.logger = logging.getLogger(__name__)
 
   def analyze_file(self, file_path: str, metric_id: str = DEFAULT_METRIC_ID):
@@ -147,6 +198,9 @@ class AnalysisResultsManager(QObject):
       self.current_worker.quit()
       self.current_worker.wait()
 
+    # A new foreground load supersedes background prefetch of the previous file.
+    self._stop_prefetch()
+
     self.analysisStarted.emit(file_path)
     self.logger.info(
       f"Queuing analysis: {os.path.basename(file_path)} ({metric.display_name})"
@@ -161,6 +215,35 @@ class AnalysisResultsManager(QObject):
   def _on_worker_completed(self, file_path: str, result: AnalysisResult):
     self.results_cache[file_path] = result
     self.analysisCompleted.emit(file_path, result)
+    # Warm the cache for the rest of the metrics so switching is instant.
+    self._start_prefetch(file_path, result)
+
+  def _start_prefetch(self, file_path: str, result: AnalysisResult):
+    """Compute the not-yet-cached metrics in the background, one at a time."""
+    self._stop_prefetch()
+    pending = [m for m in METRICS.values() if m.id not in result.metric_data]
+    if not pending:
+      return
+    self.logger.info(
+      f"Prefetching {len(pending)} metric(s) for {os.path.basename(file_path)}")
+    self.prefetch_worker = PrefetchWorker(file_path, result, pending)
+    self.prefetch_worker.computedOne.connect(self._on_prefetch_one)
+    self.prefetch_worker.start()
+
+  def _stop_prefetch(self):
+    worker = self.prefetch_worker
+    if worker is not None and worker.isRunning():
+      worker.stop()
+      worker.wait()
+    self.prefetch_worker = None
+
+  def _on_prefetch_one(self, file_path: str, metric_id: str, data: object):
+    result = self.results_cache.get(file_path)
+    if result is not None and metric_id not in result.metric_data:
+      result.metric_data[metric_id] = data
+    # metricReady (not metricTiming): warms any waiting view without spamming the
+    # status bar with background completions.
+    self.metricReady.emit(file_path, metric_id)
 
   def request_metric(self, file_path: str, metric_id: str) -> bool:
     """Ensure the metric's data exists for the file; emit metricReady when ready.
@@ -198,12 +281,13 @@ class AnalysisResultsManager(QObject):
     worker.start()
     return True
 
-  def _on_metric_completed(self, file_path: str, metric_id: str, data: object):
+  def _on_metric_completed(self, file_path: str, metric_id: str, data: object, seconds: float):
     result = self.results_cache.get(file_path)
     if result is not None:
       result.metric_data[metric_id] = data
     self.metric_workers.pop((file_path, metric_id), None)
     self.metricReady.emit(file_path, metric_id)
+    self.metricTiming.emit(file_path, metric_id, seconds)
 
   def _on_metric_failed(self, file_path: str, metric_id: str, error_message: str):
     self.metric_workers.pop((file_path, metric_id), None)
@@ -241,3 +325,14 @@ class AnalysisResultsManager(QObject):
 
   def is_file_analyzed(self, file_path: str) -> bool:
     return file_path in self.results_cache
+
+  def shutdown(self):
+    """Stop all background threads cleanly (call on app close)."""
+    self._stop_prefetch()
+    if self.current_worker and self.current_worker.isRunning():
+      self.current_worker.quit()
+      self.current_worker.wait()
+    for worker in list(self.metric_workers.values()):
+      if worker.isRunning():
+        worker.wait()
+    self.metric_workers.clear()
diff --git a/main.py b/main.py
index 08d1afb..c8bb5d3 100644
--- a/main.py
+++ b/main.py
@@ -121,8 +121,14 @@ class MainWindow(QMainWindow):
         self.analysis_manager.metricComputeStarted.connect(self.on_metric_compute_started)
         self.analysis_manager.metricReady.connect(self.on_metric_ready)
         self.analysis_manager.metricComputeError.connect(self.on_metric_compute_error)
+        self.analysis_manager.metricTiming.connect(self.on_metric_timing)
         self.visualization_widget.referenceLineMoved.connect(self.on_reference_line_moved)
     
+    def closeEvent(self, event):
+        """Stop background analysis/prefetch threads before the window closes."""
+        self.analysis_manager.shutdown()
+        super().closeEvent(event)
+
     def dragEnterEvent(self, event):
         """Handle drag enter event for file drops."""
         if event.mimeData().hasUrls():
@@ -199,9 +205,13 @@ class MainWindow(QMainWindow):
         self.visualization_widget.set_status(f"Error analyzing {filename}: {error_message}")
     
     def on_progress_update(self, message, percentage):
-        """Called when analysis progress updates."""
+        """Called when analysis progress updates.
+
+        The messages already carry phase + timing; the percentage was a coarse
+        fake (load jumped 10->done), so it's logged but not shown in the slip.
+        """
         self.logger.debug(f"Progress: {message} ({percentage}%)")
-        self.visualization_widget.set_status(f"{message} ({percentage}%)")
+        self.visualization_widget.set_status(message)
     
     def on_file_selected(self, item):
         """Called when a file is highlighted (drives the metadata panel only)."""
@@ -296,6 +306,16 @@ class MainWindow(QMainWindow):
             return  # no longer part of the overlay set
         self._refresh_view()
 
+    def on_metric_timing(self, file_path: str, metric_id: str, seconds: float):
+        """An on-demand metric compute finished — report how long it took."""
+        if file_path not in self._overlay_paths():
+            return
+        if metric_id != self.plot_control.current_metric_id():
+            return
+        metric = METRICS.get(metric_id)
+        display = metric.display_name if metric else metric_id
+        self.visualization_widget.set_status(f"{display} computed in {seconds:.1f}s")
+
     def on_metric_compute_error(self, file_path: str, metric_id: str, error_message: str):
         self.logger.error(f"Metric compute failed ({metric_id} / {os.path.basename(file_path)}): {error_message}")
         if file_path in self._overlay_paths():
diff --git a/metrics.py b/metrics.py
index cc92102..c26088c 100644
--- a/metrics.py
+++ b/metrics.py
@@ -17,7 +17,6 @@ the renderer, applied uniformly to every metric.
 
 from __future__ import annotations
 
-import warnings
 from abc import ABC, abstractmethod
 from typing import Any
 
@@ -61,18 +60,95 @@ def _window_peaks(abs_signal: np.ndarray, starts: np.ndarray, window_n: int) ->
   return running[centers]
 
 
+# BS.1770 loudness offset and absolute gate, shared by the routines below.
+_LUFS_OFFSET = -0.691
+_ABS_GATE = -70.0
+
+
+def _kweight(audio_file: AudioFile) -> np.ndarray:
+  """K-weighted mono signal (float64), filtered once and cached on the AudioFile.
+
+  Uses pyloudnorm's own BS.1770 biquad coefficients and filtering (passband_gain
+  * lfilter, exactly as `IIRfilter.apply_filter`), so every loudness quantity
+  derived from it matches pyloudnorm. Depends on `Meter._filters` internals; the
+  dev-time validation guards against a coefficient change.
+  """
+  cached = getattr(audio_file, "_yk", None)
+  if cached is not None:
+    return cached
+  yk = audio_file.y_mono.astype(np.float64, copy=False)
+  for filt in pyln.Meter(audio_file.sr)._filters.values():
+    yk = filt.passband_gain * scipy_signal.lfilter(filt.b, filt.a, yk)
+  audio_file._yk = yk
+  return yk
+
+
+def _block_loudness(yk: np.ndarray, sr: int, block_s: float, step_pct: float):
+  """Per-block mean-square energy `z` and block loudness `l`, matching pyloudnorm.
+
+  Blocks are `block_s` long, stepped by `block_s * step_pct`; energy is divided
+  by the *nominal* block length (not the rounded sample count), exactly as
+  BS.1770 / pyloudnorm define it.
+  """
+  T = len(yk) / sr
+  n_blocks = int(np.round((T - block_s) / (block_s * step_pct)) + 1)
+  if n_blocks < 1:
+    return np.array([]), np.array([])
+  j = np.arange(n_blocks)
+  lo = (block_s * (j * step_pct) * sr).astype(int)
+  up = np.minimum((block_s * (j * step_pct + 1) * sr).astype(int), len(yk))
+  csq = np.concatenate(([0.0], np.cumsum(yk * yk)))
+  z = (csq[up] - csq[lo]) / (block_s * sr)
+  with np.errstate(divide="ignore"):
+    l = _LUFS_OFFSET + 10.0 * np.log10(z)
+  return z, l
+
+
+def _integrated_lufs(yk: np.ndarray, sr: int) -> float:
+  """ITU-R BS.1770 integrated (two-stage gated) loudness from the K-weighted signal.
+
+  Reimplements pyloudnorm's gating on 400 ms / 75%-overlap blocks — validated
+  bit-equal to `Meter.integrated_loudness` — so the whole-signal re-filter that
+  pyloudnorm would do is avoided (the K-weighting is already cached).
+  """
+  z, l = _block_loudness(yk, sr, block_s=0.4, step_pct=0.25)
+  abs_gated = l >= _ABS_GATE
+  if not abs_gated.any():
+    return float("-inf")
+  gamma_r = _LUFS_OFFSET + 10.0 * np.log10(np.mean(z[abs_gated])) - 10.0
+  gated = (l > gamma_r) & (l > _ABS_GATE)
+  if not gated.any():
+    return float("-inf")
+  return float(_LUFS_OFFSET + 10.0 * np.log10(np.mean(z[gated])))
+
+
+def _loudness_range(yk: np.ndarray, sr: int) -> float:
+  """EBU Tech 3342 loudness range (LU) from the K-weighted signal.
+
+  3 s blocks at ~10 Hz with 1.5 s of trailing silence, absolute + relative
+  gating, then the 95th-minus-10th percentile spread — matching pyloudnorm's
+  `loudness_range` (validated bit-equal).
+  """
+  yk_padded = np.concatenate((yk, np.zeros(int(1.5 * sr))))
+  _, l = _block_loudness(yk_padded, sr, block_s=3.0, step_pct=0.03)
+  abs_gated = l[l >= _ABS_GATE]
+  if len(abs_gated) == 0:
+    return float("nan")
+  stl_integrated = 10.0 * np.log10(np.mean(np.power(10.0, abs_gated / 10.0)))
+  rel_gated = abs_gated[abs_gated >= stl_integrated - 20.0]
+  if len(rel_gated) == 0:
+    return float("nan")
+  return float(np.percentile(rel_gated, 95) - np.percentile(rel_gated, 10))
+
+
 def _short_term_lufs(audio_file: AudioFile, window_s: float, hop_s: float):
   """True (ungated) EBU R128 short-term loudness series + window-centre times.
 
-  K-weights the whole signal *once* with pyloudnorm's own BS.1770 biquad
-  coefficients, then takes a vectorised sliding mean-square. This is ~8x faster
+  A vectorised sliding mean-square over the cached K-weighted signal — ~8x faster
   than the old loop of per-window `integrated_loudness` calls, which also wrongly
-  gated each 3 s window — short-term loudness is ungated by definition. The
-  integrated number and LRA (which *are* gated) still come from pyloudnorm.
+  gated each 3 s window (short-term loudness is ungated by definition).
 
-  Memoised on the AudioFile so LUFS and PSR (same 3 s / 0.5 s window) share one
-  computation. Depends on pyloudnorm's `Meter._filters` internals; the dev-time
-  validation against pyloudnorm guards against a coefficient change.
+  Memoised on the AudioFile so LUFS and PSR (same 3 s / 0.5 s window) share it.
   """
   key = (round(window_s, 6), round(hop_s, 6))
   cache = getattr(audio_file, "_st_lufs_cache", None)
@@ -81,24 +157,20 @@ def _short_term_lufs(audio_file: AudioFile, window_s: float, hop_s: float):
   if key in cache:
     return cache[key]
 
-  y = audio_file.y_mono.astype(np.float64, copy=False)
+  yk = _kweight(audio_file)
   sr = audio_file.sr
-  meter = pyln.Meter(sr)
-  yk = y
-  for filt in meter._filters.values():
-    yk = scipy_signal.lfilter(filt.b, filt.a, yk) * filt.passband_gain
-
+  n = len(yk)
   window_n = max(int(window_s * sr), 1)
   hop_n = max(int(hop_s * sr), 1)
-  if len(y) < window_n:
-    ms = float(np.mean(yk * yk)) if len(yk) else 0.0
-    times = np.array([len(y) / (2.0 * sr)])
-    lufs = np.array([-0.691 + 10.0 * np.log10(max(ms, _EPS))])
+  if n < window_n:
+    ms = float(np.mean(yk * yk)) if n else 0.0
+    times = np.array([n / (2.0 * sr)])
+    lufs = np.array([_LUFS_OFFSET + 10.0 * np.log10(max(ms, _EPS))])
   else:
     csq = np.concatenate(([0.0], np.cumsum(yk * yk)))
-    starts = _window_starts(len(y), window_n, hop_n)
+    starts = _window_starts(n, window_n, hop_n)
     ms = (csq[starts + window_n] - csq[starts]) / window_n
-    lufs = -0.691 + 10.0 * np.log10(np.maximum(ms, _EPS))
+    lufs = _LUFS_OFFSET + 10.0 * np.log10(np.maximum(ms, _EPS))
     times = (starts + window_n / 2.0) / sr
 
   cache[key] = (times, lufs)
@@ -208,7 +280,6 @@ class LUFSMetric(Metric):
   SILENCE_FLOOR = -70.0  # BS.1770 absolute gate
 
   def compute(self, audio_file: AudioFile):
-    y = audio_file.y_mono.astype(np.float64, copy=False)
     sr = audio_file.sr
 
     # Short-term series: fast, ungated, shared with PSR.
@@ -216,18 +287,10 @@ class LUFSMetric(Metric):
     lufs = np.clip(np.where(np.isfinite(lufs), lufs, self.SILENCE_FLOOR),
                    self.SILENCE_FLOOR, 0.0)
 
-    # Integrated loudness + LRA keep pyloudnorm's exact gating (one call each).
-    meter = pyln.Meter(sr)
-    with warnings.catch_warnings():
-      warnings.simplefilter("ignore")
-      integrated = self._safe_integrated(meter, y)
-      if len(y) >= int(self.WINDOW_S * sr):
-        try:
-          lra = float(meter.loudness_range(y))
-        except (ValueError, FloatingPointError):
-          lra = float("nan")
-      else:
-        lra = float("nan")
+    # Integrated + LRA from the same cached K-weighting (gating matches pyloudnorm).
+    yk = _kweight(audio_file)
+    integrated = _integrated_lufs(yk, sr)
+    lra = _loudness_range(yk, sr) if len(yk) >= int(self.WINDOW_S * sr) else float("nan")
 
     return {
       "times": times,
@@ -236,13 +299,6 @@ class LUFSMetric(Metric):
       "lra": lra,
     }
 
-  @staticmethod
-  def _safe_integrated(meter: "pyln.Meter", segment: np.ndarray) -> float:
-    try:
-      return float(meter.integrated_loudness(segment))
-    except (ValueError, FloatingPointError):
-      return float("-inf")
-
   def build_spec(self, data, view=DEFAULT_VIEW) -> PlotSpec:
     times = data["times"]
     lufs = data["lufs"]