diff --git a/kidlisp/PIECE-IL.md b/kidlisp/PIECE-IL.md
index 22e485c4d5..dbd23a7fb6 100644
--- a/kidlisp/PIECE-IL.md
+++ b/kidlisp/PIECE-IL.md
@@ -234,6 +234,13 @@ The Wasm and JavaScript numbers agree bit for bit on every operation
 as the raytrace backends are. A host without WebAssembly runs the
 JavaScript; a host without a GPU runs the Wasm; the piece does not change.
 
+`(gpu kernel from [to])` is the same map dispatched to the device as a
+compute pass: the results land in the pool on the next frame that calls
+it, one frame late by design, which the source says by using `gpu` and not
+`run`. Without WebGPU, or while the device warms up, `gpu` is `run`. A
+readback costs about a millisecond regardless of size, so `gpu` pays only
+for pools in the thousands; `run` on Wasm is right for a hundred points.
+
 What this is not: the whole program. Control flow over many small
 polygons, the painter's order, the near-plane clipper with its variable
 output stay in the closure compiler. Kernels take the parts that are
diff --git a/kidlisp/tools/bench-pieces.mjs b/kidlisp/tools/bench-pieces.mjs
index 58119d254a..b6af3a90ba 100644
--- a/kidlisp/tools/bench-pieces.mjs
+++ b/kidlisp/tools/bench-pieces.mjs
@@ -63,7 +63,7 @@ async function measure(browser, file, [width, height]) {
     samples: fps.length, fpsMedian: sorted[Math.floor(sorted.length / 2)] ?? 0, fpsMin: sorted[0] ?? 0, fpsMax: sorted.at(-1) ?? 0, errors, shot};
 }
 
-const browser = await puppeteer.launch({headless: true, executablePath: CHROME, args: ['--no-sandbox', '--ignore-certificate-errors', '--disable-background-timer-throttling', '--disable-renderer-backgrounding', '--disable-backgrounding-occluded-windows', '--autoplay-policy=no-user-gesture-required', '--mute-audio']});
+const browser = await puppeteer.launch({headless: true, executablePath: CHROME, args: ['--no-sandbox', '--ignore-certificate-errors', '--enable-unsafe-webgpu', '--enable-features=WebGPU', '--use-angle=metal', '--disable-background-timer-throttling', '--disable-renderer-backgrounding', '--disable-backgrounding-occluded-windows', '--autoplay-policy=no-user-gesture-required', '--mute-audio']});
 const results = [];
 try {
   for (const file of files) for (const size of SIZES) {