> And—yes—you can read this as my being unlikely to use Rails the next time I'm building a traditional multi-page application.
Really? The way I do it is I write a client library for the API and run the tests in Rspec on top of the client library. Sure I'm going through the whole stack, but the thing operates as fast as you can serve normal users! Surely your large project can handle 5000 concurrent requests? While that might seem like a lot of buildout (scalable testing servers), I'd argue the opposite is a lot of buildout: writing an application without Rails.
Granted my approach might be most feasible for Rails API + Ember, but I really don't see the speed of my tests passing as what's holding me back.
The other thing that is nice is that once you start needing to speed up endpoints by pulling them into a compiled language, you can keep your tests! Everything is going through the API anyway! So feel free to Nim or Rust the CPU heavy endpoint or Redis-cache the write-once-in-a-while-and-keep endpoint.