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A workaround is to use close_fds=False which is only safe if all code used in Python implements the PEP 446.
This PEP mentions the FreeBSD issue, mentioning bpo-11284 with MAXFD=655,000:
"The operation can be slow if MAXFD is large. For example, on a FreeBSD buildbot with MAXFD=655,000, the operation took 300 ms: see issue #11284: slow close file descriptors."
Note: the subprocess module is now able to use posix_spawn() syscall, but it doesn't use it if close_fds=True, so it's outside the scope of this issue.