diff --git a/docs/SCROLL_PERFORMANCE.md b/docs/SCROLL_PERFORMANCE.md
index 904191df..67788396 100644
--- a/docs/SCROLL_PERFORMANCE.md
+++ b/docs/SCROLL_PERFORMANCE.md
@@ -349,6 +349,15 @@ A soak is only meaningful against a fixed workload. Compare runs with the same
 content and `--preview` setting, and alternate which build goes first when you
 A/B two of them. A live-API workload drifts over time.
 
+The soak says how often; the service's log says why. A scroll that presents no
+frame for 250 ms logs `Render stall:` with the stack of the render thread and
+the top of every other thread's, and whether the whole interpreter was blocked
+(C code holding the GIL) rather than one thread. To see what is behind the
+shorter hitches, run the service with `LEDMATRIX_STALL_WATCHDOG_MS=30`, which
+dumps at three refreshes late instead: its extra polling costs a little GIL
+time of its own, so do that on a diagnostic run, not a soak you are grading.
+`LEDMATRIX_STALL_WATCHDOG=0` turns it off.
+
 ### Results: hdpi, 2026-09-24
 
 Pi 4, 4×128×64 on one chain (512×64), `gpio_slowdown` 3, cap 120 Hz, the
diff --git a/src/common/frame_timing.py b/src/common/frame_timing.py
index 3e94b6fe..2c151f3a 100644
--- a/src/common/frame_timing.py
+++ b/src/common/frame_timing.py
@@ -62,7 +62,10 @@ top of every other thread's, so the log names what the render thread was
 waiting on. It also measures how late its own wake-up was: if the watchdog was
 held up as long as the render thread, the whole interpreter was blocked (C
 code holding the GIL, or the process not scheduled), not one thread on a lock.
-Set ``LEDMATRIX_STALL_WATCHDOG=0`` to turn it off.
+Set ``LEDMATRIX_STALL_WATCHDOG=0`` to turn it off, or
+``LEDMATRIX_STALL_WATCHDOG_MS`` to dump at a lower threshold -- 30 catches
+frames three refreshes late, which is where GIL contention shows. It polls
+three times per threshold, so keep it to diagnostic runs, not soaks.
 """
 
 from __future__ import annotations
@@ -283,7 +286,7 @@ class FrameTimingRecorder:
             self._static_frames += 1
         elif self.watchdog is None and self.scrolling_now is not None \
                 and os.environ.get("LEDMATRIX_STALL_WATCHDOG", "1") != "0":
-            self.watchdog = StallWatchdog(self)
+            self.watchdog = StallWatchdog(self, **watchdog_settings())
             self.watchdog.start()
         elif previous is not None and previous[1]:
             interval = presented_at - previous[0]
@@ -417,6 +420,23 @@ class FrameTimingRecorder:
             raise
 
 
+def watchdog_settings() -> Dict[str, float]:
+    """StallWatchdog arguments from ``LEDMATRIX_STALL_WATCHDOG_MS``, if set.
+
+    The poll comes down with the threshold, or a stall shorter than one poll
+    would go unseen.
+    """
+    try:
+        ms = float(os.environ.get("LEDMATRIX_STALL_WATCHDOG_MS") or 0)
+    except ValueError:
+        ms = 0.0
+    if ms <= 0:
+        return {}
+    threshold = ms / 1000.0
+    return {"threshold": threshold,
+            "poll": min(WATCHDOG_POLL_SECONDS, threshold / 3)}
+
+
 class StallWatchdog:
     """Log what the render thread is doing when a scroll stops presenting.
 
diff --git a/test/test_frame_timing.py b/test/test_frame_timing.py
index 274bf029..c2f27b62 100644
--- a/test/test_frame_timing.py
+++ b/test/test_frame_timing.py
@@ -9,6 +9,8 @@ import sys
 import time
 from pathlib import Path
 
+import pytest
+
 sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
 from src.common import frame_timing  # noqa: E402
@@ -303,6 +305,24 @@ def test_watchdog_rate_limits_its_dumps(caplog):
     assert dog.stalls == 2
 
 
+def test_watchdog_threshold_can_be_lowered_for_a_diagnostic_run(monkeypatch):
+    monkeypatch.delenv("LEDMATRIX_STALL_WATCHDOG_MS", raising=False)
+    assert frame_timing.watchdog_settings() == {}
+    monkeypatch.setenv("LEDMATRIX_STALL_WATCHDOG_MS", "nonsense")
+    assert frame_timing.watchdog_settings() == {}
+    monkeypatch.setenv("LEDMATRIX_STALL_WATCHDOG_MS", "30")
+    settings = frame_timing.watchdog_settings()
+    assert settings["threshold"] == pytest.approx(0.030)
+    assert settings["poll"] == pytest.approx(0.010)   # sees a stall one poll long
+
+    # ...and the recorder starts its watchdog with them.
+    rec = frame_timing.FrameTimingRecorder(path=None)
+    rec.scrolling_now = lambda: True
+    monkeypatch.setattr(frame_timing.StallWatchdog, "start", lambda self: None)
+    rec.record(0.001, 0.009, 1, True, 1.0)
+    assert rec.watchdog.threshold == pytest.approx(0.030)
+
+
 # --- measuring the panel, and runs that never locked -------------------------
 # The refresh measurement and the "not locked" cases came from the first
 # version of scripts/render_bench.py, which graded runs with its own module.