Don't confuse being IO bound for not having CPU overhead. Being IO bound just means that your app is going to hit a concurrency ceiling due to IO before you hit full saturation on all your cores. It does not mean that a given request can't be faster. Despite the fact that our apps aren't CPU-bound, a few years ago, I patched some performance problems out of Haml and shaved multiple dozens of milliseconds off of each of our requests. That translates into faster page renders, snappier responses, and happier users.

While faster view renders might not improve your theoretical maximum throughput, it will certainly have a positive impact on your users' experience.