Building a project isn't a single-core workload usually, though.

Isn't C++ compilation mostly single-core-bound still? Of course it does compile as many files in parallel as there are cores, but it's not possible to compile a single file in multiple threads.

Yeah but most projects that have compile times long enough to care about have many source files.

You haven't seen most Android projects that have a massive legacy module that Gradle spends 5 minutes compiling and doing nothing else. Seeing those 7 lines say IDLE is a daily experience

There is a lot more in single core performance then just clock speed, in fact ipc is even more important then clock speed when it comes to single core computing