I've started to notice that too. I used to average 15mph but would get to work breathing hard and covered in sweat. I now average about 12mph and am ready to work after a few minutes and a change of clothes.

The thing is, my time is still improving. I think before I was really pumping it on flats and then struggling up hills. Now I am much more consistent in my speeds, keeping it around 15mph on flats and 10mph up hills. Average speed down (my computer doesn't measure when I'm stopped), but I get to work 10 minutes sooner.