A lot of it is Android, yes. A regular build run will involve kotlin annotation processing, kotlin compilation, java annotation processing, java compilation, AAPT2, dexing, sometimes d8/r8 if you're in release mode, as well as a whole host of various processes that you can forget about until it hits you with a build error that will have you puzzled for a few hours.

EDIT: right, Jetifier too, because that's always a lot of fun.

From experience, non-android java project will build roughly in the same time as comparable amount of C# code, with both maven and gradle. It is definitely android that increases it.