Rails can serve data far in excess of 15RPS. I've built apps that serve plenty of responses in <15ms. That's one app server on one CPU core. Of course, if you do a lot in a given request it's pretty easy to get that number down as low as you want.
We serve millions of requests per day and have some slow responses (75+ ms) but on any given day our servers handle 175 requests per second without breaking a sweat. =/
So for example if an app does something bad like performing a full table scan on every request against a 300 million collection, so every request to that backend is timing out at 30 seconds, and there are thousands of them per second, well -- pretty soon your fixed pool will be full of requests timing out to that backend.