Speed up and re-enable (un-@Nightly) tests #15308
Description
Description
See #15212 for background.
The "long tail" of slowest tests was marked @Nightly with a comment of // incredibly slow or similar with 5 commits:
This is not ideal: just a workaround to help keep the CI going. Instead we would prefer tests run quickly so that they can e.g. run when testing new PRs.
We should fix the performance issues with these tests if possible. Maybe some truly need to remain @Nightly, but it should be the last resort. In many cases, the test might just be slow "some of the time" or consume too many resources (threads). For example, multithreaded cases using more than 2 threads: it should not be needed. Other tests might be using too much data/memory/cpu cycles and just need to be toned down. Other ones might even be real performance issues, we have to look into them more to figure out.
Feel free to open issue/PR for any of these tests if you have an idea how to fix them.