Upgrade to a beefier machine so builds finish faster. (However, the dashboard shows about 30% CPU usage and 20% RAM usage, so I'm not sure this is the problem...)
Send a status code right away and send the body later. Downside: we'd no longer be able to give an accurate failure vs. not-failure status code, so we break pure HTTP semantics.
PR preview often fails with 524 Origin Time-out. I believe this is because CloudFlare, which sits in front of our new build.whatwg.org, has a short-ish timeout. https://community.cloudflare.com/t/community-tip-fixing-error-524-a-timeout-occurred/42342 implies it's 100 seconds.
Potential solutions:
Other ideas also welcome.