PSA: If you are on the Heroku Common runtime then it is 100% worth your time testing and verifying Router 2 isn't going to impact performance, especially if you use puma. Router 1 is EOL and Heroku plan to start force-migrating standard/performance tier apps in a couple weeks and there is still technically a bug in Puma relating to the keep-alive setting!

See https://www.heroku.com/blog/pumas-routers-keepalives-ohmy/ for the bug and https://help.heroku.com/JJ3M1TOM/common-runtime-legacy-router-end-of-life-faq for the current published EOL timeline.