I helped write what might be a similar API gateway prototype in rails (made a DB query and decorated downstream requests with auth/role/etc headers). You might have threadpool/process/concurrency tuning to do. Even then, I don't think rails is a good solution for what is essentially an HTTP proxy, and I would've pushed for a different tool if time wasn't an issue. Rails was fast to build, I'll give it that. We ended up getting downstream service to $1M ARR so rails wins out against my "let me write it in XYZ language/framework" again somehow.
This is the point I wanted to make. Rails is great for some problems, not great at others. I can tell you for example that it's not great at transforming 2MB of JSON into a differently structured 2MB of JSON.
