The fact that zsh is a parent doesn't necessarily hurt performance. The parent and child can share the stdin fd, and the child could be the only one reading from it.
As an example of what I was referring to, in zsh, this
a >&2 | b
a | tee /dev/stderr | b
That is, zsh inserted itself into the middle of the pipeline. There are two pipes instead of one; zsh/tee reads from the a pipe, and then writes that data to the b pipe (and stderr). This does hurt performance.
-> % python test.py 2>&1 | python test.py
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
[snip]
python 24150 someone 0u CHR 136,0 0t0 3 /dev/pts/0
python 24150 someone 1w FIFO 0,8 0t0 72772851 pipe
python 24150 someone 2w FIFO 0,8 0t0 72772851 pipe
python 24150 someone 3w CHR 5,0 0t0 1049 /dev/tty
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
[snip]
python 24151 someone 0r FIFO 0,8 0t0 72772851 pipe
python 24151 someone 1u CHR 136,0 0t0 3 /dev/pts/0
python 24151 someone 2u CHR 136,0 0t0 3 /dev/pts/0
python 24151 someone 3w CHR 5,0 0t0 1049 /dev/tty
someone@arch-two [01:29:00] [~]
-> % echo $ZSH_VERSION
5.2
someone@arch-two [01:29:16] [~]
-> % cat test.py
import os
import sys
import subprocess
with open("/dev/tty", "w") as f:
subprocess.check_call(["lsof", "-p", str(os.getpid())], stdout=f)
It's not particularly surprising that the zsh behavior is different than bash; zsh redirections are only Bourne shell-like in simple cases.
I dislike zsh's behavior because it isn't what the user typed. It should be simple to look at a command line and see how many pipes it makes; see the processes that will be involved in the pipeline. Zsh's behavior seems to me to be clever and implicit; I'd much rather have dumb and explicit.