Someone's gotta explain to me how and why oh-my-zsh is slow. I've never once experienced it being slow for me. In fact, I switched to prezto and it was unbelievably slow. I switched back to oh-my-zsh. Maybe I'm just not using it right...
If you clear out your logfiles by trashing /private/var/log/asl/*.asl, there's a pretty decent speedup.
Do you have any insight as to why this happens? It smells like some kind of log parsing to present messages on login, but I've never received any such messages, and a quick vgrep of the various profile/rc's never turned anything up.