The slowness of my zsh prompt when in a git-svn managed directory was killing me. I improved it by removing the git status stuff that slows it down...

git_prompt_info.zsh

function git_prompt_info() {
  ref=$(git symbolic-ref HEAD 2> /dev/null) || return
  echo "$ZSH_THEME_GIT_PROMPT_PREFIX${ref#refs/heads/}$ZSH_THEME_GIT_PROMPT_SUFFIX"
}

If someone uses gruvbox theme on ZSH, you may have to comment out prompt_bzr from build_prompt function in ~/.oh-my-zsh/custom/themes/gruvbox.zsh-theme file.

Or you may check why ZSH lags by executing this

zsh -vx

this saved my day!

Thanks ; would it be enough if I just paste it into .zshrc file?

Awesome and thanks @liladas!
@avatar-lavventura, add this snippet just above the source $ZSH/oh-my-zsh.sh line in your ~/.zshrc

function git_prompt_info() {
  local ref
  if [[ "$(command git config --get oh-my-zsh.hide-dirty)" != "1" ]]; then
    if [[ "$(__git_prompt_git config --get oh-my-zsh.hide-status 2>/dev/null)" != "1" ]]; then
      ref=$(__git_prompt_git symbolic-ref HEAD 2> /dev/null) || \
      ref=$(__git_prompt_git rev-parse --short HEAD 2> /dev/null) || return 0
      echo "$ZSH_THEME_GIT_PROMPT_PREFIX${ref#refs/heads/}$(parse_git_dirty)$ZSH_THEME_GIT_PROMPT_SUFFIX"
    fi
  fi
}

And inside your desired (probably very large) git repo, run this command git config --add oh-my-zsh.hide-dirty 1