> After upgrading the library, performance and CPU characteristics on Scala 3 became indistinguishable from Scala 2.13.

We had a similar experience moving Ruby 2->3, which has a ton of performance improvements. It was in fact faster in many ways but we had issues with RAM spiking in production where it didn't in the past. It turned out simply upgrading a couple old dependencies (gems) to latest versions fixed most of the issues as people spotted similar issues as OP.

It's never good enough just to get it running with old code/dependencies, always lots of small things that can turn into bigger issues. You'll always be upgrading the system, not just the language.