250k/s is roughly the same speed as context switching, so while slow for pure computation, it is a reasonable amount of "waste" for switching between concurrent tasks.

If you didn't prevent preemptive context switches during your benchmarking, it's entirely possible the only thing you measured was the context switch time.

This is a fun experiment, but to get a rigorous idea of the overhead involved takes more work than what anyone in the post or comments has done.