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.
