Twitter search - 3x faster by replacing ruby on rails with java

"In the next phase of our deploy, we will eliminate Ruby on Rails entirely, connecting users directly to Blender and potentially reducing latencies even further."

Obviously for an app the size of Twitter, cutting CPU load and search latencies even a small percentage offers significant monetary savings, but I'm curious how to actually calculate the return on investment of this sort of improvement, specifically how "faster search returns for the user" should be account for, or other UX benefits.

Thoughts?