Build system performance tends to be IO-bound, but responsiveness for things like no-op builds tends to be CPU-bound -- and the JVM in particular really suffers. Optimizing IO performance makes it perform better, while optimizing CPU performance makes it feel better to use.

Arguably, making tools feel better to use is more important than raw performance. When you're doing a no-op build, the difference between 100 milliseconds and 500 is large, and the difference between 10ms and 100 is enormous.