…unless your tests are syscall heavy, then it can be 10-100× slower :(. It also doesn't support SMP. I really wish for a better version of this, we could really use it, especially since the "time stops while CPU is in use" part also means your tests won't randomly fail if the host is heavily loaded and things take longer. Unfortunately, doing something about it is outside my area of expertise.
It's testing. Using timetravel mode you can skip sleeps and speedup your unit tests massively.