A shortcut to getting dictionary definitions, which might help said definition pop up faster, is to put the mouse cursor over the word in question, and press Ctrl-Command-D. Usually works close-enough-to-instantaneous for me.
Ooh - just realised that if you keep Ctrl and Command held down, you can move the cursor around the screen and it'll try to define every word it comes across. Not sure how useful that is, but it's looks kinda cool :)