use /bin/sh instead of login shell
When launching post-processing command, you use a login shell.
On my system: /usr/local/bin/zsh --login -c '...'
Login shell is *slow*. It has fancy prompt, auto-completion, start-up subscripts, aliases and overrides, etc. It's used for interactive sessions. It's not needed for running scripts. It takes ~5s to start on my system. Fine for a terminal session, no good for script which runs every time i press »Save».
Please use /bin/sh instead.