Serve stale pages while the initial synchronous revalidation is attempted

This would improve the caching, although I believe it's even better to make the s-maxage very large (only affects CDNs), and purge the cache each time the website is rebuilt by a webhook.

stale-while-revalidate indicates the asset and can be served stale for up to an additional 300 seconds to parallel requests for the same resource while the initial synchronous revalidation is attempted.

https://support.cloudflare.com/hc/en-us/articles/115003206852-Understanding-Origin-Cache-Control