It wouldn't be fair to say zsh is slow because ohmyzsh is slow. ohmyzsh is notorious for being a bit bloated. If you pull the whole thing, it makes a mess of your shell and you really can't tell anymore what is what.

It's possible to install the individual stuff you need from oh my zsh without pulling the whole thing.

I am a happy antidote user. With it, you can do something like this on your zsh_plugins.txt file:

ohmyzsh/ohmyzsh path:plugins/extract

Though ohmyzsh provides its own means to enable and disable plugins, this will allow you to cut that down to the pulling only of individual plugins in the first place.

Your mileage may vary, but other plugin managers may give you different ways to accomplish the same.

zsh is quite an advanced shell. You will find other shells that do things radically different and have their own bells and whistles, but if you are going for feature parity it may be hard to find a replacement.