There definitely are. I've spent a lot of time with UBT, and a "reasonable" amount of time with cmake and friends. UBT isn't quite the same as CMake + Ninja. UBT does "adaptive" unity builds, globbing, and a couple of other things.

> but I don’t think it would save more than 30% here and there.

Agreed. The clean build with UBT is painfully slow compared to Cmake + Ninja, but the full builds themselves are pretty good, and I'd bet that there's probably less low hanging fruit there.

I did a good chunk of work on improving compile times in Unreal, and there is definitely just low hanging fruit in the engine for improving compile times. Some changes to how UHT works around forward declares would also make a significant difference too.