perf: lazy-load MDX images and serve them as WebP

docs.config.js

@@ -1,5 +1,11 @@
 const config = {
-	DOCS_LATEST_VERSION: '7.7'
+	DOCS_LATEST_VERSION: '7.7',
+	// Remote image origins that Next's optimizer may fetch, convert to WebP and cache.
+	OPTIMIZED_IMAGE_ORIGINS: [
+		'https://sourcegraphstatic.com/',
+		'https://storage.googleapis.com/sourcegraph-assets/',
+		'https://storage.googleapis.com/changelog-static-assets-prod/'
+	]
 };
 
 module.exports = config;

next.config.js

@@ -1,4 +1,5 @@
 const {withContentlayer} = require('next-contentlayer2');
+const {OPTIMIZED_IMAGE_ORIGINS} = require('./docs.config');
 /** @type {import('next').NextConfig} */
 
 // in prod, we serve the docs from sourcegraph.com/docs, and this requires special config on the GFE side
@@ -18,6 +19,12 @@ const nextConfig = {
 	env: {
 		NEXT_PUBLIC_DOCS_BASE_PATH: basePath
 	},
+	// MDX screenshots are uncompressed PNGs on GCS with a one-hour cache. ZoomableImage
+	// routes them through `/_next/image`, which serves WebP from Vercel's edge cache.
+	images: {
+		remotePatterns: OPTIMIZED_IMAGE_ORIGINS.map(origin => new URL(`${origin}**`)),
+		minimumCacheTTL: 60 * 60 * 24 * 7
+	},
 	// With basePath set, nothing serves `/`, so the *.vercel.app deployment URL
 	// that Vercel links from Slack / GitHub 404s. sourcegraph.com never proxies
 	// `/` to us, so this only affects visitors opening that raw URL. Stay on the

src/components/mdx/ZoomableImage.tsx

@@ -2,9 +2,23 @@
 
 import {useCallback, useEffect, useState} from 'react';
 
+import docsConfig from '../../../docs.config';
+
 interface ZoomableImageProps extends React.ImgHTMLAttributes<HTMLImageElement> {}
 
-export function ZoomableImage({className, alt, ...props}: ZoomableImageProps) {
+/**
+ * Serve allow-listed remote PNG/JPEGs through Next's image optimizer: WebP, at most
+ * 1920px wide (enough for the content column on a 2x display), cached at the edge.
+ * The width must be one of Next's default `deviceSizes`.
+ */
+function optimizedSrc(src: ZoomableImageProps['src']) {
+	if (typeof src !== 'string' || !/\.(png|jpe?g)$/i.test(src)) return src;
+	if (!docsConfig.OPTIMIZED_IMAGE_ORIGINS.some(origin => src.startsWith(origin))) return src;
+	const basePath = process.env.NEXT_PUBLIC_DOCS_BASE_PATH ?? '';
+	return `${basePath}/_next/image?url=${encodeURIComponent(src)}&w=1920&q=75`;
+}
+
+export function ZoomableImage({className, alt, src, ...props}: ZoomableImageProps) {
 	const [isOpen, setIsOpen] = useState(false);
 
 	const openModal = useCallback(() => setIsOpen(true), []);
@@ -32,6 +46,9 @@ export function ZoomableImage({className, alt, ...props}: ZoomableImageProps) {
 			<img
 				className={`cursor-zoom-in rounded-xl ${className ?? ''}`}
 				alt={alt}
+				src={optimizedSrc(src)}
+				loading="lazy"
+				decoding="async"
 				onClick={openModal}
 				{...props}
 			/>
@@ -68,6 +85,7 @@ export function ZoomableImage({className, alt, ...props}: ZoomableImageProps) {
 					<img
 						className="max-h-[90vh] max-w-[90vw] cursor-zoom-out rounded-lg object-contain"
 						alt={alt}
+						src={src}
 						onClick={e => e.stopPropagation()}
 						{...props}
 					/>