diff --git a/engine/MUNGA/JMOVER.cpp b/engine/MUNGA/JMOVER.cpp
index ae2c912..5fad336 100644
--- a/engine/MUNGA/JMOVER.cpp
+++ b/engine/MUNGA/JMOVER.cpp
@@ -117,6 +117,16 @@ EntitySegment*
 }
 
 //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// #149 perf telemetry (BT_PERF_LOG; counters cost two increments when unset).
+// gBTSegWDirty counts the EXPENSIVE arm -- the mark-every-segment pass that
+// invalidates the whole table -- separately from plain calls, because the #141
+// sweep put this accessor on per-frame weapon/beam paths and the open question
+// is whether that multiplied the dirty-pass rate (each pass forces the next
+// query of EVERY segment to re-derive its parent chain).
+int    gBTSegWCalls = 0;
+int    gBTSegWDirty = 0;
+double gBTSegWMs    = 0.0;
+
 //
 void
 	JointedMover::GetSegmentToWorld(
@@ -125,6 +135,10 @@ void
 	)
 {
 	Check(this);
+	static const int sPerf = getenv("BT_PERF_LOG") ? 1 : 0;
+	LARGE_INTEGER t0, t1, fq;
+	++gBTSegWCalls;
+	if (sPerf) QueryPerformanceCounter(&t0);
 	JointSubsystem *joints = GetJointSubsystem();
 	Check(joints);
 
@@ -135,6 +149,7 @@ void
 	//
 	if (joints->AreJointsModified())
 	{
+		++gBTSegWDirty;
 		EntitySegment::SegmentTableIterator iterator(segmentTable);
 		EntitySegment *current_segment;
 		while( (current_segment = iterator.ReadAndNext() ) != NULL)
@@ -153,6 +168,12 @@ void
 		my_segment.GetSegmentToEntity(),
 		localToWorld
 	);
+	if (sPerf)
+	{
+		QueryPerformanceCounter(&t1);
+		QueryPerformanceFrequency(&fq);
+		gBTSegWMs += 1000.0 * (double)(t1.QuadPart - t0.QuadPart) / (double)fq.QuadPart;
+	}
 	Check_Fpu();
 }
 
diff --git a/engine/MUNGA_L4/L4VIDEO.cpp b/engine/MUNGA_L4/L4VIDEO.cpp
index 4fd3a02..a8d118a 100644
--- a/engine/MUNGA_L4/L4VIDEO.cpp
+++ b/engine/MUNGA_L4/L4VIDEO.cpp
@@ -9034,6 +9034,20 @@ void DPLRenderer::ExecuteImplementation(RendererComplexity, RendererOrigin::Inte
 			DEBUG_STREAM << "[rstat] frames=" << sFrames << " avg=" << (sAcc / sFrames)
 				<< "ms maxDraw=" << sMaxD << " maxPresent=" << sMaxP
 				<< " batches=" << gNumBatches << " culled=" << gBTNumCulled << "\n" << std::flush;
+			// #149: segment-refresh telemetry on the same cadence (BT_PERF_LOG).
+			// calls = GetSegmentToWorld entries; dirty = the mark-every-segment
+			// invalidation passes (the expensive arm the #141 sweep may have
+			// multiplied); ms = time inside the accessor for the whole window.
+			{
+				static const int sSegPerf = getenv("BT_PERF_LOG") ? 1 : 0;
+				extern int gBTSegWCalls, gBTSegWDirty;
+				extern double gBTSegWMs;
+				if (sSegPerf)
+					DEBUG_STREAM << "[segperf] calls=" << gBTSegWCalls
+						<< " dirty=" << gBTSegWDirty
+						<< " ms=" << gBTSegWMs << "\n" << std::flush;
+				gBTSegWCalls = 0; gBTSegWDirty = 0; gBTSegWMs = 0.0;
+			}
 			sAcc = 0.0; sFrames = 0; sMaxD = 0.0; sMaxP = 0.0;
 		}
 	}
diff --git a/game/reconstructed/mech4.cpp b/game/reconstructed/mech4.cpp
index 6667676..222b880 100644
--- a/game/reconstructed/mech4.cpp
+++ b/game/reconstructed/mech4.cpp
@@ -9031,8 +9031,20 @@ void
 					// This is the BEAM muzzle -- the same stale-cache exposure the
 					// missile launch had, so a peer's beam would also originate
 					// from the bind-pose gun port instead of the twisted torso.
+					// #149 A/B (BT_BEAM_SEGFRESH=0): revert THIS site to the
+					// pre-sweep plain compose, to measure whether the per-beam
+					// per-frame dirty-pass is the 857 draw-stall regression.
+					// This site runs inside the DRAW path per emitter per frame;
+					// the other swept sites are per-salvo/per-hit and cannot be
+					// a per-frame cost.  Default = fresh (the swept behaviour).
+					static const int sBeamFresh =
+						!(getenv("BT_BEAM_SEGFRESH") && *getenv("BT_BEAM_SEGFRESH") == '0');
 					LinearMatrix mw;
-					GetSegmentToWorld(*s_portCache[energyOrdinal], &mw);
+					if (sBeamFresh)
+						GetSegmentToWorld(*s_portCache[energyOrdinal], &mw);
+					else
+						mw.Multiply(s_portCache[energyOrdinal]->GetSegmentToEntity(),
+							localToWorld);
 					mz = mw;					// Point3D = matrix translation
 				}
 			}
diff --git a/scratchpad/night14/segperf.sh b/scratchpad/night14/segperf.sh
new file mode 100644
index 0000000..a139461
--- /dev/null
+++ b/scratchpad/night14/segperf.sh
@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+# =========================================================================
+# #149 -- 817 -> 857 stall regression (Oracle: 97 blocks >50ms, all in the
+# mission-load window, maxDraw up to 518ms; zero such blocks on 817, same
+# machine).  My filed suspect was the #141 segment-cache sweep -- but that is
+# a HYPOTHESIS, and #137 just taught us what plausible-but-unmeasured
+# theories are worth.  So: measure first.
+#
+# INSTRUMENTATION (this build):
+#   [segperf] calls=<GetSegmentToWorld entries> dirty=<mark-every-segment
+#   invalidation passes> ms=<time inside the accessor>   -- per rstat window,
+#   BT_PERF_LOG-gated.  The DIRTY count is the tell: if the beam path
+#   multiplies invalidations, dirty >> mechs-per-frame and ms tracks maxDraw.
+#
+# A/B: identical 2-node runs, beams firing continuously.
+#   run A: default            (swept behaviour: beam muzzle via GetSegmentToWorld)
+#   run B: BT_BEAM_SEGFRESH=0 (pre-sweep plain compose at the BEAM site only)
+# If A's stalls/dirty collapse in B, the beam-site sweep is the regression.
+# If A == B and both are clean, the suspect is EXONERATED and the real 857
+# load-stall cause is elsewhere (next: diff the load path).
+# =========================================================================
+set -x
+. /c/git/bt411/scratchpad/night6/bench_common.sh
+cd /c/git/bt411/content || exit 1
+
+run_one () {   # $1 = tag, $2 = extra env value for BT_BEAM_SEGFRESH
+  taskkill //F //IM btl4.exe > /dev/null 2>&1
+  sleep 2
+  rm -f sp_${1}_a.log sp_${1}_b.log sp_${1}_relay.log
+  bt_expert_egg MP.EGG SP.EGG
+  sed -i "s/^map=.*/map=grass/; s/^time=.*/time=day/; s/^vehicle=.*/vehicle=madcat/" SP.EGG
+  ( export BT_GOTO=enemy BT_GOTO_STOP=120 BT_AUTOFIRE=1 BT_AF_PERIOD=3
+    export BT_MP_LOG=1
+    bt_launch sp_${1}_b.log SP.EGG 0x0C -net 1601 )
+  sleep 2
+  ( export BT_GOTO=enemy BT_GOTO_STOP=120 BT_AUTOFIRE=1 BT_AF_PERIOD=3
+    export BT_PERF_LOG=1 BT_MP_LOG=1
+    export BT_BEAM_SEGFRESH=${2}
+    bt_launch sp_${1}_a.log SP.EGG 0x03 -net 1501 )
+  sleep 5
+  python ../tools/btconsole.py SP.EGG 127.0.0.1:1501 127.0.0.1:1601 > sp_${1}_relay.log 2>&1 &
+  local RELAY=$!
+  sleep 260
+  kill $RELAY 2>/dev/null
+  sleep 3
+  bt_kill_ours; sleep 2; taskkill //F //IM btl4.exe > /dev/null 2>&1; sleep 3
+}
+
+run_one fresh 1
+run_one legacy 0
+
+echo "=================== #149 SEGPERF A/B ==================="
+for t in fresh legacy; do
+  echo "--- $t ---"
+  python - <<PY
+import re, io
+v=[]; seg=[]
+for ln in io.open(r"C:\git\bt411\content\sp_${t}_a.log", encoding="latin-1", errors="replace"):
+    m=re.search(r"\[rstat\] frames=\d+ avg=([\d.]+)ms maxDraw=([\d.]+)", ln)
+    if m: v.append((float(m.group(1)), float(m.group(2))))
+    m=re.search(r"\[segperf\] calls=(\d+) dirty=(\d+) ms=([\d.]+)", ln)
+    if m: seg.append((int(m.group(1)), int(m.group(2)), float(m.group(3))))
+if v:
+    import statistics
+    avgs=[a for a,_ in v]; draws=[d for _,d in v]
+    stall=sum(1 for a,_ in v if a>50)
+    print("  rstat blocks=%d  avg-med=%.2fms  maxDraw-med=%.2f  maxDraw-worst=%.1f  blocks>50ms=%d"
+          % (len(v), statistics.median(avgs), statistics.median(draws), max(draws), stall))
+if seg:
+    c=[x[0] for x in seg]; d=[x[1] for x in seg]; ms=[x[2] for x in seg]
+    print("  segperf windows=%d  calls-med=%d  dirty-med=%d  ms-med=%.2f  ms-worst=%.2f"
+          % (len(seg), sorted(c)[len(c)//2], sorted(d)[len(d)//2], sorted(ms)[len(ms)//2], max(ms)))
+PY
+done