> Opening a new shell can take ~5s with oh-my-zsh

Heh. I remember when zsh first got its fancy completion system, compinit would add so long to zsh startup on the dinky little machines of the day that people literally started adding progress bars and the like to their zshrc.

I stuck with the old-style completion for a long time.

Hm. I'll take a look at Prezto then. For me a new shell takes approx .5 seconds and tab completion is instant, but I'm on a 2014 MacBook Pro and use iTerm2 instead of the native terminal (not sure if that makes any difference). I don't recall it being any worse on the 2012 MacBook Air + iTerm2 I had previously, but maybe I just didn't know any better.