I'm slower in the rain most of the time for two reasons. First of all, I also slow down when its wet because I've put it down a couple of times taking turns on wet surfaces. Hitting the ground sucks. The second reason is that, for me anyway, the rain is usually accompanied by a headwind, at least on the longer ride home. The winds in this area come from the south and can be somewhat strong. Yesterday it wasn't too bad -- a little misty/light rain with a 5-10 MPH headwind.