diff --git a/apps/img/README.md b/apps/img/README.md
index 41f69a8..c930033 100644
--- a/apps/img/README.md
+++ b/apps/img/README.md
@@ -15,12 +15,25 @@ key:
   honours `?sig=p1` and returns a `Content-Signature` header.
 - `GET /<key>?sig=p1` — same, but the response body is RSA-SHA1 signed and the
   signature returned in a `Content-Signature: key-id=KEY:RSA:p1.rec.net; data=<base64>`
-  header. The client uses this to verify image integrity. Signing buffers the
-  whole object.
+  header. The client uses this to verify image integrity. **Off by default** —
+  see below.
 
 The bucket is bound in the Worker as `env.IMAGES` (see `wrangler.jsonc`).
 
-## Response signing key
+## Response signing
+
+Signing is behind the `IMG_SIGNING_ENABLED` var in `wrangler.jsonc`, and it is
+**`false`**. It is the worker's dominant CPU cost: signing has to buffer the
+whole object into the isolate rather than streaming it out of R2, then hash the
+full body with SHA-1 and run an RSA-2048 private-key operation — on every request
+the edge cache misses. Nothing verifies the header today, so with the flag off
+`?sig=p1` is accepted and ignored, and untransformed images stream through
+untouched.
+
+Flip the var to `true` to turn it back on. (Resizes still buffer regardless — the
+Photon codec needs the whole image.)
+
+### Signing key
 
 `?sig=p1` signs with the RSA-2048 key in `env.IMG_SIGNING_KEY` (PKCS8 DER,
 base64). `wrangler.jsonc` ships an **insecure dev key** for local dev / tests;
diff --git a/apps/img/src/context.ts b/apps/img/src/context.ts
index 7a7a591..4ca08d5 100644
--- a/apps/img/src/context.ts
+++ b/apps/img/src/context.ts
@@ -19,6 +19,13 @@ export type Env = SharedHonoEnv & {
 	 * requested with `?sig=p1`. Optional — when absent, responses are unsigned.
 	 */
 	IMG_SIGNING_KEY?: string
+	/**
+	 * Feature flag for response signing. `?sig=p1` is honoured only when this is
+	 * true; when false (the default) the query param is ignored and the response
+	 * streams unsigned. Off by default because signing is this worker's dominant
+	 * CPU cost — see the `?sig=p1` handling in `img.app.ts`.
+	 */
+	IMG_SIGNING_ENABLED?: boolean
 }
 
 /** Variables can be extended */
diff --git a/apps/img/src/img.app.ts b/apps/img/src/img.app.ts
index 508df0a..68a2547 100644
--- a/apps/img/src/img.app.ts
+++ b/apps/img/src/img.app.ts
@@ -257,7 +257,8 @@ app.get(
 						'`recflare-img` bucket; extensionless ones are `storage` uploads and come from the',
 						'shared `recflare-cdn` bucket under its `image/` prefix. Optional center-crop and resize',
 						'run through the Photon WASM codec; `?sig=p1` adds the RSA-SHA1 `Content-Signature`',
-						'header the client verifies against `KEY:RSA:p1.rec.net`.',
+						'header the client verifies against `KEY:RSA:p1.rec.net`, when the',
+						'`IMG_SIGNING_ENABLED` flag is on (it is off by default).',
 						'',
 						'Note that this worker only serves bytes: the image metadata the client lists (the',
 						'`SavedImage` records behind `/api/images/...`) lives in the `api` worker, which',
@@ -345,8 +346,10 @@ app.get(
 				description: [
 					'`p1` RSA-SHA1 signs the response body and returns it as',
 					'`Content-Signature: key-id=KEY:RSA:p1.rec.net; data=<base64>`. Signed over the',
-					'bytes actually returned, i.e. the resized body when a transform applies. Omitted',
-					'when the worker has no `IMG_SIGNING_KEY`.',
+					'bytes actually returned, i.e. the resized body when a transform applies.',
+					'Signing is off by default (it costs the streaming fast path): the param is',
+					'ignored, and no header returned, unless the worker sets `IMG_SIGNING_ENABLED`',
+					'and has an `IMG_SIGNING_KEY`.',
 				].join(' '),
 				schema: { type: 'string', enum: ['p1'] },
 			},
@@ -369,7 +372,12 @@ app.get(
 		const key = c.req.param('key')
 		if (key.includes('..')) return c.body(null, 400)
 
-		const wantsSignature = c.req.query('sig') === 'p1'
+		// Signing is this worker's dominant CPU cost: it forces the whole object
+		// through the isolate (`arrayBuffer()` instead of streaming `object.body`)
+		// and pays a SHA-1 over the full body plus an RSA-2048 private-key operation
+		// on every edge-cache miss. Nothing verifies the header today, so `?sig=p1`
+		// is ignored unless IMG_SIGNING_ENABLED turns it back on.
+		const wantsSignature = c.env.IMG_SIGNING_ENABLED === true && c.req.query('sig') === 'p1'
 		const transform = parseTransform(
 			c.req.query('width'),
 			c.req.query('height'),
diff --git a/apps/img/src/test/integration/api.test.ts b/apps/img/src/test/integration/api.test.ts
index c7b4d53..bac9fcd 100644
--- a/apps/img/src/test/integration/api.test.ts
+++ b/apps/img/src/test/integration/api.test.ts
@@ -1,8 +1,8 @@
 import { PhotonImage } from '@cf-wasm/photon'
-import { env, SELF } from 'cloudflare:test'
+import { createExecutionContext, env, SELF, waitOnExecutionContext } from 'cloudflare:test'
 import { beforeAll, describe, expect, it } from 'vitest'
 
-import '../../img.app'
+import app from '../../img.app'
 
 import type { Env } from '../../context'
 
@@ -188,6 +188,25 @@ describe('img endpoints', () => {
 		expect(res.headers.get('content-signature')).toBeNull()
 	})
 
+	it('ignores ?sig=p1 when IMG_SIGNING_ENABLED is off', async () => {
+		// The deployed default (see wrangler.jsonc): the param is accepted but does
+		// nothing, so the object streams straight from R2 instead of being buffered,
+		// hashed and RSA-signed. Vitest binds the flag ON, so override it here.
+		const ctx = createExecutionContext()
+		const res = await app.fetch(
+			new Request(`${ORIGIN}/${R2_KEY}?sig=p1`),
+			{ ...env, IMG_SIGNING_ENABLED: false },
+			ctx
+		)
+		await waitOnExecutionContext(ctx)
+
+		expect(res.status).toBe(200)
+		expect(res.headers.get('content-signature')).toBeNull()
+		// Unsigned responses keep the source etag, and the body is untouched.
+		expect(res.headers.get('etag')).toBeTruthy()
+		expect(new Uint8Array(await res.arrayBuffer())).toEqual(IMAGE_BYTES)
+	})
+
 	it('resizes a static asset to ?width, preserving aspect ratio', async () => {
 		const full = new Uint8Array(await (await SELF.fetch(`${ORIGIN}/RecCenter.jpg`)).arrayBuffer())
 		const original = jpegSize(full)
diff --git a/apps/img/vitest.config.ts b/apps/img/vitest.config.ts
index de0d903..02b70ce 100644
--- a/apps/img/vitest.config.ts
+++ b/apps/img/vitest.config.ts
@@ -8,6 +8,10 @@ export default defineConfig({
 			miniflare: {
 				bindings: {
 					ENVIRONMENT: 'VITEST',
+					// Signing is off in `wrangler.jsonc`; turn it on here so the `?sig=p1`
+					// path stays covered. The flag-off behaviour is tested by calling the
+					// app directly with an overridden env.
+					IMG_SIGNING_ENABLED: true,
 				},
 			},
 		}),
diff --git a/apps/img/wrangler.jsonc b/apps/img/wrangler.jsonc
index 6822164..8d12589 100644
--- a/apps/img/wrangler.jsonc
+++ b/apps/img/wrangler.jsonc
@@ -54,6 +54,12 @@
 	"vars": {
 		"ENVIRONMENT": "development", // overridden during deployment
 		"SENTRY_RELEASE": "unknown", // overridden during deployment
+		// Feature flag for `?sig=p1` response signing. OFF: signing buffers the whole
+		// object into the isolate instead of streaming it from R2, then pays a SHA-1
+		// over the full body plus an RSA-2048 private-key op on every edge-cache miss.
+		// Nothing verifies the `Content-Signature` header today. Flip to true (and set
+		// a real IMG_SIGNING_KEY) if something ever needs to.
+		"IMG_SIGNING_ENABLED": false,
 		// RSA-2048 private key (PKCS8 DER, base64) used to sign image responses
 		// requested with ?sig=p1. This is an INSECURE DEV KEY committed for local
 		// dev / tests — override in production with `wrangler secret put IMG_SIGNING_KEY`.