Is Rails as fast as .NET?
Right now though, the dev ergonomics and productivity of .NET core are not actually very far behind rails at all, so you can serve your pages in 2ms instead of 200ms without suffering for it.
In some regards actually, perf helps a lot. Running a large bank of tests for example - those are going to complete a LOT more quickly in .NET which means you can iterate faster and have a better dev experience