To print Vim's startup time:

$ vim --startuptime /dev/stdout +qall

Note that this was tested on Vim 7.3.

If you have compiled Vim with profiling enabled (brew install macvim doesn't):

$ vim --version | grep +profile

Then you can profile Vim commands as described in this article: https://coderwall.com/p/sdva9q/how-to-detect-plugins-slowing-vim-down