Does anyone know how it compare to zsh with oh-my-zsh?
Fish "just works", and you won't have any lag. It is a delightful beginner experience, too.
A Zsh veteran could probably replicate parts of the Fish experience in Zsh, but ultimately they are different tools. I use Fish at home for casual shell stuff, but I write shell scripts in Zsh, and I use Zsh at work whenever possible.