OpenJDK’s build tools have an auto tuning feature for setting parallel jobs. It caps at 16. I’ve found this both too low for builds (I have 24 cores) and too high for tests (I have 32G RAM, and test instances of Java are configured with 512M of heap by default).

That said the build tooling around OpenJDK is really well thought out. I think my machines profile is just not the right “shape” for the defaults