Add FAQ about lag.

README.mkd

@@ -165,6 +165,15 @@ Your colorscheme is configuring the `SignColumn` highlight group weirdly.  Pleas
 
 This happens on certain combinations of OS and Vim.  You can prevent the flicker by adding `let g:gitgutter_sign_column_always = 1` to your `~/.vimrc`.
 
+> There's a noticeable lag when vim-gitter runs; how can I avoid it?
+
+By default vim-gitgutter runs often so the signs are as accurate as possible.  However on some systems this causes a noticeable lag.  If you would like to trade a little accuracy for speed, add one or both of these to your ~/.vimrc`:
+
+```
+let g:gitgutter_on_bufenter = 0
+let g:gitgutter_all_on_focusgained = 0
+```
+
 > Why is no sign shown if I delete the first line(s) in a file?
 
 vim-gitgutter shows removed lines with a sign on the line above.  In this case there isn't a line above so vim-gitgutter can't show the sign.