Parallelizing builds and tests is definitely useful. Not only does it make things run faster but it can force concurrency issues to the surface that you would otherwise not find until after you deploy the code. This works especially well with integration tests that each might take a few seconds to run. Most of that is IO so you can actually utilize more threads than CPU cores here and still expect a speedup.

However, it requires designing your tests such you can do this. One key trick is using randomized data and test fixtures. Another one is avoiding the overhead of initializing databases, etc. over and over again. Of course, all that stuff is useful in just about any language. But some languages make this easier than others.