A single core machine already overloaded is going to get even worse introducing the cpu overhead of gzipping response bodies (assuming it’s cpu bound and not IO bound)

Cache control headers will help with return traffic

More cpu cores

If using nginx ensure sendfile is enabled and workers are set to auto or tuned for your setup

Check ulimit file handle limits

Offload static assets to cdn

Since it’s a static html site, you could even host on s3, netlify, etc