sbt / scalatest , setting parallelism

I would like to control the number of threads that sbt uses when running in parallel. One reason is that a lot of my tests are calling rest api's which means they are waiting most of the time, so if I increase the threads above the number of cpu's, the tests should run faster.

I've seen a lot of configurations that claim to do this, i.e.

concurrentRestrictions in Test := Tags.limit(Tags.CPU,16) :: Nil

But nothing seems to make any difference. Using sbt 1.4.3