Although it is a RoR app you still can cache a lot of thing out of the box. As others have pointed out, just using cloudflare as CDN would already help somewhat.

Recently I've seen a blog post about a company using RoR for generating a completely static website. Unfortunately I can't find it anymore.

The naive approach would be to use "wget -r" a locally running and "rsync" the generated html to the server but there might be some gotchas with that.