diff --git a/packages/cli/src/browser/gpuPolicy.colorGradingStall.test.ts b/packages/cli/src/browser/gpuPolicy.colorGradingStall.test.ts
new file mode 100644
index 0000000000..a95fa75c38
--- /dev/null
+++ b/packages/cli/src/browser/gpuPolicy.colorGradingStall.test.ts
@@ -0,0 +1,71 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const mocks = vi.hoisted(() => ({
+  ensureBrowser: vi.fn(),
+  resolveBrowserGpuMode: vi.fn(async (): Promise<"hardware" | "software"> => "hardware"),
+}));
+
+vi.mock("./manager.js", () => ({
+  ensureBrowser: mocks.ensureBrowser,
+}));
+
+vi.mock("@hyperframes/engine", () => ({
+  resolveBrowserGpuMode: mocks.resolveBrowserGpuMode,
+}));
+
+import { detectColorGradingGpuStallRisk } from "./gpuPolicy.js";
+
+const GRADED_HTML =
+  '<div data-composition-id="main"><img data-color-grading=\'{"adjust":{"saturation":-1}}\' src="a.jpg" /></div>';
+const UNGRADED_HTML = '<div data-composition-id="main"><img src="a.jpg" /></div>';
+
+describe("detectColorGradingGpuStallRisk", () => {
+  beforeEach(() => {
+    vi.resetAllMocks();
+    mocks.ensureBrowser.mockResolvedValue({ executablePath: "/chrome", source: "cache" });
+  });
+
+  afterEach(() => {
+    vi.resetAllMocks();
+  });
+
+  it("warns when the composition uses color grading and no hardware GPU is found", async () => {
+    mocks.resolveBrowserGpuMode.mockResolvedValue("software");
+    const warning = await detectColorGradingGpuStallRisk(GRADED_HTML, "auto");
+    expect(warning).toContain("data-color-grading");
+    expect(warning).toContain("SwiftShader");
+    expect(warning).toContain("--timeout");
+  });
+
+  it("stays silent when a real hardware GPU is found", async () => {
+    mocks.resolveBrowserGpuMode.mockResolvedValue("hardware");
+    const warning = await detectColorGradingGpuStallRisk(GRADED_HTML, "auto");
+    expect(warning).toBeNull();
+  });
+
+  it("stays silent (and never probes) when the composition has no color grading", async () => {
+    const warning = await detectColorGradingGpuStallRisk(UNGRADED_HTML, "auto");
+    expect(warning).toBeNull();
+    expect(mocks.ensureBrowser).not.toHaveBeenCalled();
+    expect(mocks.resolveBrowserGpuMode).not.toHaveBeenCalled();
+  });
+
+  it("stays silent (and never probes) when software mode was explicitly requested", async () => {
+    const warning = await detectColorGradingGpuStallRisk(GRADED_HTML, "software");
+    expect(warning).toBeNull();
+    expect(mocks.ensureBrowser).not.toHaveBeenCalled();
+    expect(mocks.resolveBrowserGpuMode).not.toHaveBeenCalled();
+  });
+
+  it("forces an 'auto' probe even for an explicit --browser-gpu request, to get the ground truth", async () => {
+    mocks.resolveBrowserGpuMode.mockResolvedValue("software");
+    await detectColorGradingGpuStallRisk(GRADED_HTML, "hardware");
+    expect(mocks.resolveBrowserGpuMode).toHaveBeenCalledWith("auto", { chromePath: "/chrome" });
+  });
+
+  it("treats a probe failure as nothing to warn about", async () => {
+    mocks.resolveBrowserGpuMode.mockRejectedValue(new Error("probe boom"));
+    const warning = await detectColorGradingGpuStallRisk(GRADED_HTML, "auto");
+    expect(warning).toBeNull();
+  });
+});
diff --git a/packages/cli/src/browser/gpuPolicy.test.ts b/packages/cli/src/browser/gpuPolicy.test.ts
index 7ea3766721..e8ef9244bf 100644
--- a/packages/cli/src/browser/gpuPolicy.test.ts
+++ b/packages/cli/src/browser/gpuPolicy.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it } from "vitest";
 import {
   assertWebGpuRequirement,
   compositionRequiresWebGpu,
+  compositionUsesColorGrading,
   resolveLocalBrowserGpuMode,
 } from "./gpuPolicy.js";
 
@@ -31,4 +32,15 @@ describe("local browser GPU policy", () => {
     expect(() => assertWebGpuRequirement(html, "hardware", "hardware")).not.toThrow();
     expect(() => assertWebGpuRequirement(html, "software", "software")).not.toThrow();
   });
+
+  it("detects data-color-grading on any element, not just the composition root", () => {
+    expect(
+      compositionUsesColorGrading(
+        '<div data-composition-id="main"><img data-color-grading=\'{"adjust":{"saturation":-1}}\' src="a.jpg" /></div>',
+      ),
+    ).toBe(true);
+    expect(
+      compositionUsesColorGrading('<div data-composition-id="main"><img src="a.jpg" /></div>'),
+    ).toBe(false);
+  });
 });
diff --git a/packages/cli/src/browser/gpuPolicy.ts b/packages/cli/src/browser/gpuPolicy.ts
index 0c809e3715..0a42b16127 100644
--- a/packages/cli/src/browser/gpuPolicy.ts
+++ b/packages/cli/src/browser/gpuPolicy.ts
@@ -1,3 +1,5 @@
+import { HF_COLOR_GRADING_ATTR } from "@hyperframes/core";
+
 export type BrowserGpuMode = "auto" | "hardware" | "software";
 export type ResolvedBrowserGpuMode = Exclude<BrowserGpuMode, "auto">;
 
@@ -41,3 +43,50 @@ export function assertWebGpuRequirement(
       "use --no-browser-gpu only when intentionally testing the composition's software fallback.",
   );
 }
+
+export function compositionUsesColorGrading(html: string): boolean {
+  const escapedAttr = HF_COLOR_GRADING_ATTR.replace(/[-/\\^$*+?.()|[\]{}]/g, "\\$&");
+  return new RegExp(`\\s${escapedAttr}(?:\\s|=|>)`, "i").test(html);
+}
+
+const COLOR_GRADING_GPU_STALL_WARNING =
+  `This composition uses ${HF_COLOR_GRADING_ATTR}, but no hardware GPU was detected — ` +
+  "the browser will render on the SwiftShader/software WebGL fallback. Color grading's " +
+  "per-element canvas readback has no fast path under software WebGL: it has been measured " +
+  "at roughly 40x slower than an ungraded composition, which is easily enough to exceed the " +
+  "navigation/render-ready timeout, or to make check/render painfully slow even once past it. " +
+  "If this run is unexpectedly slow or times out, try a much larger --timeout, run on a host " +
+  "with a real GPU, or preprocess to monochrome derivatives with grading intensity 0 before " +
+  "capturing with --browser-gpu to skip the expensive per-frame grading pass entirely.";
+
+/**
+ * Preflight for a known SwiftShader limitation (not a hyperframes bug): a
+ * per-element color-grading canvas pays a synchronous GPU-stall readback cost
+ * that software WebGL has no fast path for, ~40x slower than an ungraded
+ * composition in measured practice. `requestedMode: "software"` is a
+ * deliberate, already-informed choice and is not warned about; `"auto"` /
+ * `"hardware"` both expect speed, so a silent fallback to software there is
+ * exactly the surprise this call is meant to catch before capture starts.
+ * Reuses `resolveCaptureBrowserGpuMode`'s cached probe (forcing `"auto"` to
+ * get the ground-truth answer even when the caller requested `"hardware"`,
+ * which always reports back `"hardware"` verbatim) — resolved against the
+ * same `ensureBrowser()` executable path a subsequent real launch will use,
+ * so this doesn't seed the shared cache with a different browser's probe.
+ * Best-effort: any probe failure here is treated as "nothing to warn about"
+ * rather than failing the caller — the real launch will surface a genuine
+ * browser problem on its own.
+ */
+export async function detectColorGradingGpuStallRisk(
+  html: string,
+  requestedMode: BrowserGpuMode,
+): Promise<string | null> {
+  if (requestedMode === "software" || !compositionUsesColorGrading(html)) return null;
+  try {
+    const { ensureBrowser } = await import("./manager.js");
+    const browser = await ensureBrowser();
+    const actualMode = await resolveCaptureBrowserGpuMode("auto", browser.executablePath);
+    return actualMode === "software" ? COLOR_GRADING_GPU_STALL_WARNING : null;
+  } catch {
+    return null;
+  }
+}
diff --git a/packages/cli/src/utils/checkBrowser.ts b/packages/cli/src/utils/checkBrowser.ts
index f5c00ec52a..e61d07e8a0 100644
--- a/packages/cli/src/utils/checkBrowser.ts
+++ b/packages/cli/src/utils/checkBrowser.ts
@@ -18,6 +18,7 @@ import {
   shouldIgnoreHttpError,
   shouldIgnoreRequestFailure,
 } from "../commands/validate.js";
+import { detectColorGradingGpuStallRisk } from "../browser/gpuPolicy.js";
 import { loadBrowserScript } from "../commands/layout.js";
 import { normalizeErrorMessage } from "./errorMessage.js";
 import { ambiguousIssue, type MotionFrame } from "./motionAudit.js";
@@ -156,6 +157,14 @@ export async function runBrowserCheck(
   const { bundleWithLocalizedFonts } = await import("./bundleWithLocalizedFonts.js");
   const html = await bundleWithLocalizedFonts(project.dir);
   await preResolveHostileMediaProxies(project.dir, html, options.autoProxy);
+  const requestedGpuMode = options.browserGpuMode ?? resolveCliChromeGpuMode();
+  // Printed eagerly (not just recorded as a finding) because the risk this
+  // flags is a navigation timeout — if it fires, `runBrowserCheck` throws
+  // before ever returning a report, so a finding pushed to `drafts` would be
+  // discarded along with the whole in-flight result (see runCheckPipeline's
+  // catch, which replaces browser with emptyBrowserResult() on that path).
+  const colorGradingGpuWarning = await detectColorGradingGpuStallRisk(html, requestedGpuMode);
+  if (colorGradingGpuWarning) console.warn(`\n[hyperframes] ${colorGradingGpuWarning}`);
   const server = await serveStaticProjectHtml(
     project.dir,
     html,
@@ -164,6 +173,14 @@ export async function runBrowserCheck(
     options.autoProxy,
   );
   const drafts: RuntimeDraft[] = [];
+  if (colorGradingGpuWarning) {
+    drafts.push({
+      code: "color_grading_gpu_stall_risk",
+      severity: "warning",
+      message: colorGradingGpuWarning,
+      time: 0,
+    });
+  }
   let currentTime = 0;
   let chromeBrowser: import("puppeteer-core").Browser | undefined;
 
@@ -173,7 +190,7 @@ export async function runBrowserCheck(
       navigationTimeoutMs: options.timeout,
       renderReadyTimeoutMs: options.timeout,
       renderReadyWarningSuffix: "checking the current page state",
-      browserGpuMode: options.browserGpuMode ?? resolveCliChromeGpuMode(),
+      browserGpuMode: requestedGpuMode,
       beforeNavigate: (page) => wireRuntimeListeners(page, drafts, () => currentTime),
     });
     chromeBrowser = session.browser;