Author here. Mill also helps with running tests. Apart from all tests running parallel by default with heuristics to try and maximize performance (https://mill-build.org/blog/11-jvm-test-parallelism.html), Mill also can selectively execute tests in CI based on code changes (https://mill-build.org/mill/large/selective-execution.html), and provide performance profiles so you can see what is taking up time and improve it (https://mill-build.org/mill/depth/parallelism.html#_mill_chr...). All this is built in and comes for free, with the end result that even tests can take much less time in Mill than they do in other build tools