Heroku’s deep dive into the weird behavior between Router 2.0 and Puma is one of those posts you want to bookmark and carry into every performance triage conversation. If you’ve ever seen a Rails app suddenly spike in tail response times after flipping a platform flag and thought “why is this slower with what should be better,” this is the exact kind of investigation that cuts through the noise. The short version: what looked like a router issue turned out to be a keepalive behavior in Puma interacting badly with Router 2.0 under specific conditions. The longer version shows why digging matters.