Is there any reason not to move these tests away from the cpan, dist, ext tests? and into the tests for the core? I have a smoke-me that does this, if you'd like to see how it impacts your testing performance. https://perl5.git.perl.org/perl.git/shortlog/refs/heads/smoke-me/khw-harness This two line change improved my elapsed 'make test' time by 9% on an 8 effective core system. Presumably the reason we do base first is because if it's failing, nothing is going to work. Then we do the other core tests, in parallel. Then we wait for those to finish before starting up the remainder of the tests. Those are done in alphabetical order in a given test directory, presumably because cpan tests like to have a specified order, and have named their tests accordingly. But we do the longest-running directories first. But the API tests for XS code are not even dependent on base working. But since base is fast, there's no real gain in doing them parallel with base. But there is no reason I can see, except tradition, not to do them in parallel with the rest of the core tests. I also think there should be a way to tell harness to run cpan etc in parallel with core. A lot of time seems to be wasted waiting for all of core to finish before these start. On many-core systems like dromedary, this is a significant fraction of the elapsed time. One wouldn't want this to be the default, but I think an expert-level option is called for.Thread Next