Something that I've recently added to my PHP projects is Paratest.
It adds parallel testing to PHPUnit, so your tests will be run in parallel instead of sequentially.
For the example module in my automated testing in Drupal email course, using Paratest reduces the execution time from ~16 seconds to ~8 seconds.
In a client project with 136 tests, it reduces the time from four and a half minutes to less than two minutes.
This is a big improvement just from running a single Composer command to add Paratest.
There's also an open issue for using it for Drupal core's tests, which is something I'll keep an eye on and will look to contribute to.
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