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While investigating the performance regression of Monix 2.1.x, I discovered that ForkJoinPool (which is backing Scala's own global execution context) has been migrated in the implementation of Scala 2.12 to java.util.concurrent.ForkJoinPool and that this implementation has lower throughput than the one in Scala 2.11.

The difference is quite significant and at this point I'm actually considering introducing my own implementation of ForkJoinPool in Monix, because unless I'm not horribly mistaken, this isn't a problem that the Scala developers can fix in the standard library, unless they bring back scala.concurrent.forkjoin.ForkJoinPool. Anyway, I've started this issue on my own end:
monix/monix#281 (comment)

That said, when benchmarking, it's important to not measure the wrong thing. At least the tests that are forking threads should be backed by the same thread-pool implementation, otherwise you end up measuring the performance of the underlying thread-pools. For example I don't know what FS2 does, but Scalaz definitely doesn't use by default a ForkJoinPool for its Task.fork or Task.apply.