I looked at this. On my machine I max out at about 20 child spawn/exit cycles a second, and I need to make sure that with throttling, its definitely much slower. The problem is the 20 depends on the speed of the machine, and the build vms are slower than my machine, and that with throttling, I get a fixed time-based number of spawns a second (dependent on throttleDelay, but independent of machine). So, I need to make sure that the throttle delay is <<< how fast the test machine can start/stop workers... an unknown quantity. I've spent a bunch of time increasing test timeouts across our CI recently because test were failing on CI because the VMs are slow, its very annoying, I'd like to not do to much of this, and you can see in earlier tests how I've tried to scale timeouts (crudely), so the timeout depends on the work to be done more than the speed of the machine. Soon I'll see if that was sufficient to get CI passes.

All of which is to say... I don't want to mess with this until unit test run time is large enough that its actually annoying. And 15 seconds doesn't quite hit that, IMHO.