When I do stuff like this, I just write the front app in the new/fast language, then reproxy the stuff not written in the fast language to the legacy app. You can even add middlewares or caching or metrics in the new app for requests going to the old one, as you can run (fast) code around both the request and the response.

It’s the best of both worlds - the new app gets to see all of the traffic, but doesn’t need to implement 100% of the routes. Any added to the new app can just take precedence over the old one, carving out the path-space that gets reverse proxied.

It seems like doing FFI for this is overly complex; I’d rather take the small perf hit of doing another request to a different process.

good idea, which language you choose for that ? also with frankenphp you can just write those required apis to be really fast as go extensions.