In case close_fds=True is passed to subprocess.Popen()
or its users (subprocess.call() etc), it might spend
some considerable time closing non-opened file descriptors,
as demonstrated by the following snippet from strace:

close(3)                    = -1 EBADF (Bad file descriptor)
close(5)                    = -1 EBADF (Bad file descriptor)
close(6)                    = -1 EBADF (Bad file descriptor)
close(7)                    = -1 EBADF (Bad file descriptor)
...
close(1021)                 = -1 EBADF (Bad file descriptor)
close(1022)                 = -1 EBADF (Bad file descriptor)
close(1023)                 = -1 EBADF (Bad file descriptor)

This happens because the code in _close_fds() iterates from 3 up to
MAX_FDS = os.sysconf("SC_OPEN_MAX").

Now, syscalls are cheap, but SC_OPEN_MAX (also known as RLIMIT_NOFILE
or ulimit -n) can be quite high, for example:

    $ docker run --rm python python3 -c \
            $'import os\nprint(os.sysconf("SC_OPEN_MAX"))'
    1048576

This means a million syscalls before spawning a child process, which
can result in a major delay, like 0.1s as measured on my fast and mostly
idling laptop. Here is the comparison with python3 (which does not have this problem):

    $ docker run --rm python python3 -c $'import subprocess\nimport time\ns = time.time()\nsubprocess.check_call([\'/bin/true\'], close_fds=True)\nprint(time.time() - s)\n'
    0.0009245872497558594

    $ docker run --rm python python2 -c $'import subprocess\nimport time\ns = time.time()\nsubprocess.check_call([\'/bin/true\'], close_fds=True)\nprint(time.time() - s)\n'
    0.0964419841766

Others can correct me if I'm wrong, but I'm fairly sure 2.7 isn't making changes unless they fix critical or security-related bugs.

The code here is suboptimal, but it's already been fixed in Python 3 (in bpo-8052), as part of a C accelerator module (that reduces the risk of race conditions and other conflicts your Python level fix entails). Unless someone corrects me, I'll close this as "Won't Fix".

Yeah, this is WONTFIX particularly since you can have the Python3 implementation in Python 2 easily with https://pypi.org/project/subprocess32/.