The delay in leaving insert mode used to drive me nuts in vim-powerline. I posted an issue and the author pointed me to this helpful Vim config snippet as a workaround (original link gone as the vim-powerline issue tracker has been removed, so reposting as a gist): https://gist.github.com/brendonrapp/5944296

I imagine the delay that the README for this project is referring to might be the same thing, so hopefully it can be alleviated with this same trick.

EDIT: Looks like the official Powerline docs share this little trick now: https://powerline.readthedocs.org/en/latest/tipstricks.html