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Here is a quick profile under 3.x:
Time: 10178.0 ms
 320812 function calls (320809 primitive calls) in 10.182 seconds
 Ordered by: internal time
 ncalls tottime percall cumtime percall filename:lineno(function)
 10000 5.937 0.001 5.937 0.001 {built-in method read}
 10000 1.462 0.000 1.462 0.000 {built-in method waitpid}
 10000 1.042 0.000 1.042 0.000 {built-in method fork_exec}
 10000 0.752 0.000 8.131 0.001 subprocess.py:1119(_execute_child)
 1 0.228 0.228 10.182 10.182 bench_subprocess.py:1(<module>)
 10000 0.177 0.000 8.345 0.001 subprocess.py:631(__init__)
 20000 0.079 0.000 0.079 0.000 {built-in method close}
 10000 0.061 0.000 1.584 0.000 subprocess.py:1368(wait)
So it looks like our parent Python process spends its time (read(), waitpid()) waiting for its child...