Normally this occurs because you're running a large number of "green" threads on your own scheduler which schedules onto a thread pool underneath. This is good if all your threads are small/tiny since userspace thread creation is cheaper than creating an OS thread but if your jobs are long-lived then your userspace job scheduler is really just adding additional scheduling overhead on top of the overhead that the OS already has for thread scheduling and you would have been better with OS threads. If your M:N threading requires separate stack space for each job, there can be a sizeable overhead (this is why Rust abandoned M:N threading).