Image Optimization should use the stale-while-revalidate pattern

[styfle]

Describe the feature you'd like to request

Currently, optimized images are cached on disk for a period of time indicated by the upstream Cache-Control header. This means the first request to the image could be slow but subsequent requests are fast. But when the cache expires, the next request will be slow again.

Describe the solution you'd like

We can avoid this second slowdown by using the stale-while-revalidate pattern. Once the cache expires, we can serve the cached image anyway and generate the new optimized image in the background.

Describe alternatives you've considered

We could apply the stale-while-revalidate header, but that is still experimental and doesn't work in Safari or IE. Also, Vercel strips that header so the browser will not see it.

[github-actions]

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