diff --git a/simulator/src/compositor.ts b/simulator/src/compositor.ts
index a7fd853..56064a4 100644
--- a/simulator/src/compositor.ts
+++ b/simulator/src/compositor.ts
@@ -106,7 +106,7 @@ export class WebGLCompositor {
         gl.vertexAttribPointer(positionAttrib, 2, GL.FLOAT, false, 0, 0);
     }
 
-    composite (framebuffer: Framebuffer) {
+    composite (framebuffer: Framebuffer): number {
         const gl = this.gl;
         const bytes = framebuffer.bytes;
 
@@ -115,5 +115,7 @@ export class WebGLCompositor {
 
         // Draw the fullscreen quad
         gl.drawArrays(GL.TRIANGLES, 0, 6);
+
+        return framebuffer.countChangedPixelsAndReset();
     }
 }
diff --git a/simulator/src/framebuffer.ts b/simulator/src/framebuffer.ts
index 3eaa169..80fa6ab 100644
--- a/simulator/src/framebuffer.ts
+++ b/simulator/src/framebuffer.ts
@@ -8,17 +8,27 @@ import { FONT } from "./font";
 
 export class Framebuffer {
     bytes: Uint16Array;
+    pixelsChanged: number;
 
     constructor (memory: ArrayBuffer) {
         this.bytes = new Uint16Array(memory, ADDR_FRAMEBUFFER, WIDTH * HEIGHT);
+        this.pixelsChanged = 0;
+    }
+
+    countChangedPixelsAndReset (): number {
+      const result = this.pixelsChanged;
+      this.pixelsChanged = 0;
+      return result;
     }
 
     fillScreen (color: number): void {
         this.bytes.fill(color);
+        this.pixelsChanged += this.bytes.length;
     }
 
     drawPoint (color: number, x: number, y: number) {
         this.bytes[WIDTH * y + x] = color;
+        this.pixelsChanged += 1;
     }
 
     drawPointUnclipped (color: number, x: number, y: number) {
@@ -30,6 +40,7 @@ export class Framebuffer {
     drawHLineFast(color: number, startX: number, y: number, endX: number) {
         const yOff = WIDTH * y;
         this.bytes.fill(color, yOff + startX, yOff + endX);
+        this.pixelsChanged += endX - startX;
     }
 
     drawHLineUnclipped(color: number, startX: number, y: number, endX: number) {
diff --git a/simulator/src/runtime.ts b/simulator/src/runtime.ts
index 4588db8..7bf0537 100644
--- a/simulator/src/runtime.ts
+++ b/simulator/src/runtime.ts
@@ -5,6 +5,8 @@ import { Framebuffer } from "./framebuffer";
 import { WebGLCompositor } from "./compositor";
 import { pack565, unpack565 } from "./ui/utils";
 
+const PIXELS_PER_MILLISECOND = 512; // about 25 FPS for full screen refreshes.
+
 export class Runtime {
     canvas: HTMLCanvasElement;
     memory: WebAssembly.Memory;
@@ -258,7 +260,9 @@ export class Runtime {
     }
 
     composite () {
-        this.compositor.composite(this.framebuffer);
+        const changedPixels = this.compositor.composite(this.framebuffer);
+        const screenUpdateTimeMs = changedPixels / PIXELS_PER_MILLISECOND;
+        return screenUpdateTimeMs;
     }
 }
 
diff --git a/simulator/src/ui/app.ts b/simulator/src/ui/app.ts
index 65d17be..16436d9 100644
--- a/simulator/src/ui/app.ts
+++ b/simulator/src/ui/app.ts
@@ -373,7 +373,12 @@ export class App extends LitElement {
             }
 
             if (calledUpdate) {
-                runtime.composite();
+                const extraTimePenalty = runtime.composite();
+                const screenDoneTime = timeFrameStart + extraTimePenalty;
+                if (screenDoneTime > timeNextUpdate) {
+                    console.log("throttled by pixels!");
+                    timeNextUpdate = screenDoneTime;
+                }
 
                 // if (import.meta.env.DEV) {
                 //     // FIXED(2023-12-13): Pass the correct FPS for display