diff --git a/src/app/[...parts]/_page/NpmDiff/DiffTooLarge.tsx b/src/app/[...parts]/_page/NpmDiff/DiffTooLarge.tsx
new file mode 100644
index 00000000..98b8f22f
--- /dev/null
+++ b/src/app/[...parts]/_page/NpmDiff/DiffTooLarge.tsx
@@ -0,0 +1,37 @@
+import { type ComponentProps, forwardRef } from "react";
+import Code from "^/components/ui/Code";
+import Heading from "^/components/ui/Heading";
+import Pkg from "^/components/ui/Pkg";
+import { cx } from "^/lib/cva";
+import type SimplePackageSpec from "^/lib/SimplePackageSpec";
+
+export interface DiffTooLargeProps extends ComponentProps<"section"> {
+    a: SimplePackageSpec;
+    b: SimplePackageSpec;
+    specs: [string, string];
+}
+
+const DiffTooLarge = forwardRef<HTMLElement, DiffTooLargeProps>(
+    ({ a, b, specs, className, ...props }, ref) => (
+        <section
+            className={cx("p-12 text-center", className!)}
+            ref={ref}
+            {...props}
+        >
+            <p className="text-4xl">📦🐘</p>
+            <Heading h={3} className="mb-4">
+                This diff is too large!
+            </Heading>
+            <p className="mb-4">
+                Comparing <Pkg pkg={a} /> and <Pkg pkg={b} /> took too long. You
+                can run it locally instead:
+            </p>
+            <Code>
+                npm diff --diff={specs[0]} --diff={specs[1]}
+            </Code>
+        </section>
+    ),
+);
+DiffTooLarge.displayName = "DiffTooLarge";
+
+export default DiffTooLarge;
diff --git a/src/app/[...parts]/_page/NpmDiff/NpmDiff.tsx b/src/app/[...parts]/_page/NpmDiff/NpmDiff.tsx
index 36b8587f..e87d7c36 100644
--- a/src/app/[...parts]/_page/NpmDiff/NpmDiff.tsx
+++ b/src/app/[...parts]/_page/NpmDiff/NpmDiff.tsx
@@ -8,6 +8,7 @@ import npmDiff from "^/lib/npmDiff";
 import type SimplePackageSpec from "^/lib/SimplePackageSpec";
 import countChanges from "^/lib/utils/countChanges";
 import DiffFiles from "./DiffFiles";
+import DiffTooLarge from "./DiffTooLarge";
 import NoDiff from "./NoDiff";
 import ViewTypeSwitch from "./ViewTypeSwitch";
 
@@ -26,6 +27,9 @@ const NpmDiff = async ({ a, b, specs, options }: NpmDiffProps) => {
     const result = await npmDiff(specs, options);
 
     if (!result.ok) {
+        if (result.kind === "timeout") {
+            return <DiffTooLarge a={a} b={b} specs={specs} />;
+        }
         throw new Error(result.message);
     }
 
diff --git a/src/app/api/[...parts]/route.ts b/src/app/api/[...parts]/route.ts
index 8f4b9d0b..1b363133 100644
--- a/src/app/api/[...parts]/route.ts
+++ b/src/app/api/[...parts]/route.ts
@@ -57,7 +57,12 @@ export async function GET(req: NextRequest, { params }: DiffApiContext) {
                 headers: defaultPageCachingHeaders,
             });
         } else {
-            const status = result.kind === "not-found" ? 404 : 500;
+            const status =
+                result.kind === "not-found"
+                    ? 404
+                    : result.kind === "timeout"
+                      ? 504
+                      : 500;
             return NextResponse.json(result.message, { status });
         }
     } else {
diff --git a/src/lib/npmDiff/NpmDiffError.ts b/src/lib/npmDiff/NpmDiffError.ts
index ac680335..a32c6fee 100644
--- a/src/lib/npmDiff/NpmDiffError.ts
+++ b/src/lib/npmDiff/NpmDiffError.ts
@@ -1,3 +1,4 @@
 export type NpmDiffError =
     | { kind: "not-found"; message: string }
+    | { kind: "timeout"; message: string }
     | { kind: "unknown"; message: string };
diff --git a/src/lib/npmDiff/libnpmdiff.worker.ts b/src/lib/npmDiff/libnpmdiff.worker.ts
new file mode 100644
index 00000000..463ca7f9
--- /dev/null
+++ b/src/lib/npmDiff/libnpmdiff.worker.ts
@@ -0,0 +1,41 @@
+import libnpmdiff, { type Options } from "libnpmdiff";
+import { parentPort, workerData } from "node:worker_threads";
+
+export interface LibnpmdiffWorkerData {
+    specs: [string, string];
+    options: Options;
+}
+
+export type LibnpmdiffWorkerMessage =
+    | { ok: true; diff: string }
+    | {
+          ok: false;
+          error: {
+              code?: unknown;
+              type?: unknown;
+              wanted?: unknown;
+              pkgid?: unknown;
+              message?: unknown;
+          };
+      };
+
+const { specs, options } = workerData as LibnpmdiffWorkerData;
+
+libnpmdiff(specs, options).then(
+    (diff) => {
+        parentPort?.postMessage({
+            ok: true,
+            diff,
+        } satisfies LibnpmdiffWorkerMessage);
+    },
+    (e: unknown) => {
+        const { code, type, wanted, pkgid, message } = (e ?? {}) as Record<
+            string,
+            unknown
+        >;
+        parentPort?.postMessage({
+            ok: false,
+            error: { code, type, wanted, pkgid, message },
+        } satisfies LibnpmdiffWorkerMessage);
+    },
+);
diff --git a/src/lib/npmDiff/npmDiff.ts b/src/lib/npmDiff/npmDiff.ts
index cf665c4f..2d88525b 100644
--- a/src/lib/npmDiff/npmDiff.ts
+++ b/src/lib/npmDiff/npmDiff.ts
@@ -1,7 +1,10 @@
-import libnpmdiff, { type Options } from "libnpmdiff";
+import type { Options } from "libnpmdiff";
 import { cacheLife } from "next/cache";
 import { hasErrorCode } from "^/lib/utils/hasErrorCode";
 import type { NpmDiffError } from "./NpmDiffError";
+import { LibnpmdiffTimeoutError, runLibnpmdiff } from "./runLibnpmdiff";
+
+export const NPM_DIFF_TIMEOUT_MS = 40_000;
 
 export type NpmDiffResult =
     { ok: true; diff: string } | ({ ok: false } & NpmDiffError);
@@ -29,7 +32,7 @@ async function npmDiff(
     let startTime = 0;
     try {
         startTime = Date.now();
-        const result = await libnpmdiff(specs, options);
+        const result = await runLibnpmdiff(specs, options, NPM_DIFF_TIMEOUT_MS);
 
         cacheLife("max");
 
@@ -48,6 +51,16 @@ async function npmDiff(
             duration: Date.now() - startTime,
         });
 
+        if (e instanceof LibnpmdiffTimeoutError) {
+            cacheLife("days");
+
+            return {
+                ok: false,
+                kind: "timeout",
+                message: `The diff took too long to compute. Run "npm diff --diff=${specs[0]} --diff=${specs[1]}" locally instead.`,
+            };
+        }
+
         const isEtarget = (e: unknown): e is ErrorETARGET =>
             hasErrorCode(e, "ETARGET");
         const isE404 = (e: unknown): e is Error404 => hasErrorCode(e, "E404");
diff --git a/src/lib/npmDiff/runLibnpmdiff.ts b/src/lib/npmDiff/runLibnpmdiff.ts
new file mode 100644
index 00000000..18dac517
--- /dev/null
+++ b/src/lib/npmDiff/runLibnpmdiff.ts
@@ -0,0 +1,53 @@
+import type { Options } from "libnpmdiff";
+import { Worker } from "node:worker_threads";
+import type {
+    LibnpmdiffWorkerData,
+    LibnpmdiffWorkerMessage,
+} from "./libnpmdiff.worker";
+
+export class LibnpmdiffTimeoutError extends Error {
+    constructor(timeoutMs: number) {
+        super(`libnpmdiff did not finish within ${timeoutMs}ms`);
+        this.name = "LibnpmdiffTimeoutError";
+    }
+}
+
+export function runLibnpmdiff(
+    specs: [string, string],
+    options: Options,
+    timeoutMs: number,
+): Promise<string> {
+    return new Promise((resolve, reject) => {
+        const worker = new Worker(
+            new URL("./libnpmdiff.worker.ts", import.meta.url),
+            {
+                workerData: { specs, options } satisfies LibnpmdiffWorkerData,
+            },
+        );
+
+        const timer = setTimeout(() => {
+            reject(new LibnpmdiffTimeoutError(timeoutMs));
+            void worker.terminate();
+        }, timeoutMs);
+
+        worker.once("message", (message: LibnpmdiffWorkerMessage) => {
+            clearTimeout(timer);
+            if (message.ok) {
+                resolve(message.diff);
+            } else {
+                reject(Object.assign(new Error(), message.error));
+            }
+            void worker.terminate();
+        });
+
+        worker.once("error", (e) => {
+            clearTimeout(timer);
+            reject(e);
+        });
+
+        worker.once("exit", (code) => {
+            clearTimeout(timer);
+            reject(new Error(`libnpmdiff worker exited with code ${code}`));
+        });
+    });
+}