Usually Intellij products are slow and fleet does not seem to be slow, so it is feels likely a lot of code was rewritten to make it fast.

I don't think they're slow at all. Takes a while to index when opening a new project, then everything is snappy.

My main reason for using JB is I loathe the shitty build your own ide experience of VSC. Everything is more difficult, whereas in JB everything just works.

Fleet has a "smart mode" that can load an intellij backend for IDE like features. But that only happens if you enable it. Thats why it can do Kotlin even though Kotlin doesn't have an official LSP.

Yeah I think they're just trying to get out of Swing by developing a new ui, Swing isn't that fun to develop.

Unfortunately they started building Fleet before Compose Multiplatform was ready, so now they support three UI technologies. Granted, Fleet uses the same rendering base as Compose (Skiko), but that's got to be a dead end ultimately.