This PR and #1067 are the last things needed for supporting .pyc files. You can now change sys.dont_write_bytecode to false.
This improve the import time significantly. for example:

import time

start = time.time()
import os
print(time.time() - start)

without __pycahce__: 0.272 [sec]
with __pycache__: 0.114 [sec]
*in release