Any change of a machine (bike) will introduce variables that will impact, "average," speed. Is the diameter of your wheel on the new bike a bit larger than your old one? If you switched the computer you had from one to the other, that would tend to show a slower average than the old bike, for example. Did you ride a known distance and time your ride? Things like crank length difference (which would affect your spin) are also a variable to consider. Bike fit while understood, is most critical. Even a difference of less than an inch on the seat to crank length could affect your power transfer negatively.
Just some trivia, while older heavier bikes accelerate more slowly, since the human body is the largest source of drag, at higher speeds there is no difference between a heavier bike and a newer very light bike in average speed over a fixed course. Only at the pinnacle of racing athletes do minor differences in bike mechanics influence who wins the race. For the vast majority of us bike enthusiasts, we are the engine that makes the big difference, no matter what machine we apply our power to.