perf: lazy-load MDX images and serve them as WebP - #1975
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ZoomableImage renders every doc image eagerly, so a page with several 1 MB PNGs downloads all of them before first paint.
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The docs embed ~360 PNG screenshots from GCS (sourcegraphstatic.com and storage.googleapis.com/sourcegraph-assets), uncompressed and with a one-hour cache. Route allow-listed PNG/JPEG sources through /_next/image, which converts to WebP, caps width at 1920px and caches at Vercel's edge for a week. The zoom modal keeps the original full-resolution file. Amp-Thread-ID: https://ampcode.com/threads/T-01a0ac9d-704f-73fe-8211-1b3e5840969f Co-authored-by: Amp <amp@ampcode.com>
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Problem
A HAR of
/docs/batch-changes/delete-a-batch-changeshows 1.2 MB of PNG screenshots. They are uncompressed PNGs on GCS (sourcegraphstatic.com,storage.googleapis.com/sourcegraph-assets) served withmax-age=3600and no WebP negotiation, andZoomableImagerenders them as a bare<img>that loads eagerly.Lazy loading alone does not help this page: it is 3748 px tall with images at 666 / 870 / 1515 / 2293 px, and Chrome starts fetching
loading="lazy"images 1250 px ahead of the viewport (2500 px on slow connections), so all four load anyway. Measured on the preview vs prod: identical image bytes (1215 KB), FCP/LCP within noise. On a long page (/code-navigation/auto-indexing, 16 images) lazy loading cuts 2.8 MB → 28 KB beforeload.Fix
loading="lazy" decoding="async"on the inline image. Saves bandwidth on long pages./_next/image: WebP, capped at 1920 px wide, cached at Vercel's edge for a week (minimumCacheTTL). The allow-list lives indocs.config.jssonext.config.js(images.remotePatterns) andZoomableImageshare it. SVG, GIF and other hosts (imgur, githubusercontent) pass through unchanged. The zoom modal keeps the original full-resolution file.Measured with
next starton this branch, the four images on the HAR page:/_next/imageTrade-offs
minimumCacheTTLto fall back to Next's 4 h default if that matters.w=1920is the largest of Next's defaultdeviceSizesthat still covers the content column on a 2x display; wider screenshots are downscaled.Verification
tsc,pnpm lint,pnpm buildclean. Onnext start: everysourcegraphstatic.comPNG on the page renders through/_next/imageasimage/webpwithCache-Control: public, max-age=604800; a PNG from a bucket outside the allow-list returns 400. Page renders unchanged (images at intrinsic size within the column).