Since 2019.3 we've found that running unit tests slows to a crawl as more and more tests are run. For example in a typical run from the root by the time 640 tests have been run it's taking ten seconds or more per test. But if we run those tests as a single group they only take one or two seconds to run.
Those tests are outputting a lot of logging and it appears that commenting out some of our logging improves performance.
Our CI machine which runs the tests using NUnit completes them all in just under ten minutes whereas to be honest no-one knows how long R# test runner will take because even leaving the tests running over a lunch break didn't give them enough time to complete.