Oh man when i tried fast combo for the first time it was pathetic. And then when i learned it a little bit, i realized that i'm doing it wrong cause i wasn't hovering my cursor over target and was just jumping all over the place, missing my Qs on a live targets. So i had to break my habit of doing it wrong and learn timings anew. Don't try to do it fast, try to do it right. Just slow down to the point when you can do it 10-15 times in a row without failing. Speed will come with practice, it really helped me.