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][atdc], 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][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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