I didn't notice so much of a performance difference when I switched to clipless, I'm sure I got some, but it definitely was not dramatic. However, I find clipless pedals much more comfortable, more secure, more stable. I don't like riding without them now, not because I can't go as fast or efficient, but because it just doesn't feel right. It didn't take very long using them before I felt that way, the 'stability' effect was pretty quick.