[ARandumGuy]

I think it's a matter of effort. Many performance bottlenecks only really show up when you scale up to massive levels. While it's possible to replicate that on a test environment, it takes time and effort to set that up. While doing that is worth it for some applications, I can see why the Lichess developers didn't want to bother.

[hocuspocus]

Time, effort and spare resources that Lichess had better allocate somewhere else.

If Thibault enjoys Scala 3 and feels more productive, while the overall performance on the same hardware is approximately equal, it's already a win for the project.