I ran a little comparison with both Oh-my-zsh and Prezto with roughly the same plugins: http://i.imgur.com/cDSRZuO.png. There's almost a 0.4s difference. In practice it makes for a noticeable 0.5-1s difference (with all the other stuff I have in my .zshrc).

I have a fast system (Core i5, SSD), but tab completion is often very slow.

I just checked my `oh-my-zsh/plugins` directory, and there are 173 plugins, most of which I don't need.

Should I purge them?

Edit: browsing through the (oh-my-)zsh files, I can see that it only loads the plugins found in the `plugin` environment variable, so I guess the answer is no. The only plugin enabled is `git`.

My theme is also very basic (git and mercurial status).

Any advice to improve performance?

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).

I tested opening a new iTerm 2 tab on my late-2009 MBP and it takes 3.5-4s for the prompt to appear with oh-my-zsh enabled.

I would love to know what I've got misconfigured.