I have a few large git repositories that were very slow when I accessed them from my Mac. I tried to solve the problem with different things: running
git gc, disabling plugins, running OS X maintenance with Onyx, but nothing was solving the problem. It seems that the problem is caused by OMZ reading the git status from some repositories.
Try setting the these 2 git configurations to your repository:
This is described in more detail in this post from StackOverflow.