Something I noticed after migrating to oh-my-zsh while still on a spinning drive - if you're on a mac and clear out your system logfiles (rm /private/var/log/asl/*.asl), prompts begin to appear almost instantly even with ZSH. I have no idea what is happening to cause this, but the performance difference is unmistakable.
FWIW - I'm pretty sure this is shell-independent. Happens with BASH as well.
While this looks interesting, the main premise that oh-my-zsh is slow doesn't seem to hold up for me. I have a decently quick system, but nothing extreme and I don't notice much more lag with auto completion than I did with bash or fish. And I use auto completion excessively, and I'm not sparse with my oh-my-zsh plugins: plugins=(git golang archlinux catimg common-aliases compleat dircycle dirhistory sudo systemd url…
yea oh-my-zsh isn't that slow, most people who think it is slow either have it misconfigured or have a really, really slow system
FWIW - There is a known issue with Apple's built-in-git and the Git oh-my-zsh plugin that really slows down prompt redraws. it has to do with the way the built in Git does repository polls, I believe. The solution is to install the latest Git client (preferably via Homebrew or whatever).