4.x: parallel performs poorly with 10+ parallelism

For some reason, the parallel Scrabble benchmark performs poorly when the parallelism level is 10+, for example, on my i7 8700 CPU (6 cores/12 threads):

However, my older i7 4770K processor (4 cores/8 threads) shows no such performance degradation. Neither does the reactive-streams-commons implementation (the parent of RxJava's parallel implementation) with parallelism=12. Correction: The Rsc benchmark was pinned to 8 threads and actually shows a similar inefficiency with 10+.