diff --git a/engine/MUNGA_L4/L4GLASSWIN.cpp b/engine/MUNGA_L4/L4GLASSWIN.cpp
index 8d775ba..f9b89da 100644
--- a/engine/MUNGA_L4/L4GLASSWIN.cpp
+++ b/engine/MUNGA_L4/L4GLASSWIN.cpp
@@ -96,6 +96,8 @@ struct GButton
 
 struct GWin
 {
+	double      perfMs;				// [glassperf] paint ms this window, this second
+	int         perfN;				// [glassperf] paints this second
 	const char *title;
 	HWND        hwnd;
 
@@ -1695,7 +1697,20 @@ static LRESULT CALLBACK
 	switch (message)
 	{
 	case WM_PAINT:
-		if (w != NULL) { PaintGlass(window, w); return 0; }
+		if (w != NULL)
+		{
+			// [glassperf] (#149): time EVERY panel paint.  The blit is a
+			// HALFTONE StretchDIBits whose cost is strongly driver-dependent
+			// -- the whole point is to measure it on the machine that pays it.
+			LARGE_INTEGER t0, t1, fq;
+			QueryPerformanceCounter(&t0);
+			PaintGlass(window, w);
+			QueryPerformanceCounter(&t1);
+			QueryPerformanceFrequency(&fq);
+			w->perfMs += 1000.0 * (double)(t1.QuadPart - t0.QuadPart) / (double)fq.QuadPart;
+			++w->perfN;
+			return 0;
+		}
 		break;
 
 	case WM_ERASEBKGND:
@@ -2047,6 +2062,10 @@ void
 		return;
 	sLastPaint = now;
 
+	static const int sGlassPerf =
+		!(getenv("BT_PERF_LOG") && *getenv("BT_PERF_LOG") == '0');
+	LARGE_INTEGER gt0, gt1, gfq;
+	QueryPerformanceCounter(&gt0);
 	for (int i = 0; i < gWinCount; ++i)
 	{
 		if (gWins[i].hwnd != NULL)
@@ -2055,4 +2074,31 @@ void
 			UpdateWindow(gWins[i].hwnd);		// synchronous paint, not the throttled queue
 		}
 	}
+	QueryPerformanceCounter(&gt1);
+	QueryPerformanceFrequency(&gfq);
+
+	// [glassperf] (#149): one line per second -- the tick's total synchronous
+	// cost plus each window's own paint time.  This runs INSIDE the render
+	// frame (L4VIDEO calls the tick), so on a machine where GDI serialises
+	// against D3D present, tickMs IS the per-frame tax and the per-window
+	// split names the guilty panel.  Default ON like [segperf]; BT_PERF_LOG=0
+	// opts out.
+	static double sTickMs = 0.0; static int sTicks = 0;
+	static unsigned long sLastReport = 0;
+	sTickMs += 1000.0 * (double)(gt1.QuadPart - gt0.QuadPart) / (double)gfq.QuadPart;
+	++sTicks;
+	if (sGlassPerf && now - sLastReport >= 1000)
+	{
+		sLastReport = now;
+		DEBUG_STREAM << "[glassperf] ticks=" << sTicks << " tickMs=" << sTickMs;
+		for (int i = 0; i < gWinCount; ++i)
+		{
+			if (gWins[i].perfN > 0)
+				DEBUG_STREAM << " | " << gWins[i].title
+					<< " n=" << gWins[i].perfN << " ms=" << gWins[i].perfMs;
+			gWins[i].perfMs = 0.0; gWins[i].perfN = 0;
+		}
+		DEBUG_STREAM << "\n" << std::flush;
+		sTickMs = 0.0; sTicks = 0;
+	}
 }