In case it isn't clear, prezto is a fork of oh-my-zsh which has basically been rewritten bit by bit to make everything much cleaner and saner. I highly recommend it. The history-substring-search and syntax-highlighting modules are brilliant. You will be the envy of all your friends and neighbours.

I've been using oh-my-zsh and the startup is really slow. I hear prezto solves this. How do i upgrade from oh-my to prezto? No one seems to answer that ?

I've just made the switch after finding out about prezto from this thread. The reason the upgrade process isn't documented is that, unfortunately, there isn't one. You just have to remove oh-my-zsh and install prezto, then move over any modifications you've made manually.

That said, it wasn't much work, and I've been using it for the past couple hours and I really like it!