I once upgraded from regularly riding a stock 1980 Raleigh Super Course 12 to a 2007 Trek Madone 5.2 with Ultegra Ice. At the time I was riding 200-300 miles per week.

I immediately started averaging 4-5 mph faster during rides of almost any duration of time or distance.

I blamed the bike, and specifically the stiffness. I used to be able to look down at the stays and down tube on the Super Course and see them flexing under me as I tried to sprint or climb, and I've since heard some pretty bad things about the flimsy little crank arms that came with them as well.