No change to average speed. You might notice that clipless pedals make it easier to maintain a higher cadence, which some people find to make pedaling easier. I find that it's occasionally nice to be able to pull up on the pedals to give my quads some rest. Once you're used to them, clipless pedals are also much easier to engage; no more pedaling across an intersection (or down the street) with your toe clips dragging along the ground.