Having too many threads all running at the same time can also cause a performance hit, and I don't mean hitting the OS limit on threads. The more threads you have running in parallel(remember this is considering a GIL-less setup) the more you need to context switch between the. Having fewer threads all running in an event loop allows you to manage more events with only a few threads, for example setting the number of event loop threads to the number of cores on the cpu.