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.
It's a RoR app. Content might not change but it's still an app with a database etc.