[rhdunn]

I'd suspect that it is a combination of the following:

a) a lot of existing code that needs to be untangled (e.g. moving various tasks to not depend on indices);

b) figuring out how to avoid unnecessary re-indexing (e.g. the downoadable index caches for things like Java files);

c) having to define a migration path to spread out breaking changes to give plugins enough time to adapt.

IIUC, some of the performance work with the latest version is due to parallelizing the various background processes, which uses coroutines, so you could add "implementing language, library, and tooling support to Kotlin/Java" to that list.