For me it's the little things. I use the keyboard constantly to move around my code. I switched to SublimeText2 for a couple of days and the 1 second of added time it took to move my hand to the mouse to move around or use a gui interaction really slowed me down. It really surprised me how that little bit of time was just enough to break my flow. So when I say vim makes me faster, I mean that it itself is fast enough to keep up with my train of thought.