multithreading - stackless python and multiprocessing slowdown -

i have program runs multiple tasklets using stackless python, , problem though fast, notice slowdown if start 1 or more processes run same code.

for example, using threads inside processes takes program 6 seconds complete, , if start 2 processes still takes 6 seconds complete. if use tasklets program takes 2 seconds complete, takes 4 seconds if run 2 processes simultaneously , 6 seconds if run 3 processes , etc.

my question is, normal behavior of stackless python? checked make sure tasklets in separate processes using separate schedulers , each process can see tasklets, maybe tasklets in processes still fighting on system resources? it's explanation can think of why each additional process cause slowdown when using tasklets not threads. need explain me can figure out if there's wrong code or if unavoidable issue.