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I think your analysis is wrong. These mmap() calls are for anonymous memory, most likely they are emitted by the libc's malloc() to get some memory from the kernel. In other words they will be blazingly fast.
I would suggest you try to dig deeper. For example, how much CPU time does the parent process take (excluding its children).
Of course, I also disagree with the idea that spawning "exit 0" subprocesses is a performance critical operation ;) Therefore, it would be useful to know the absolute overhead difference (in milliseconds) between subprocess and os.popen(), to decide if there is really a problem.