The power of your shot depends more on racquet acceleration than on racquet speed. The racquet should actually be going a lot faster AFTER the contact was made.
If you were going, for example, 5 miles an hour with your racquet just before the hit, accelerating it to 20 miles per hour through and well after the hit will give you more power than going from 10 or 15 to 20 miles an hour.
This is the secret of players with short backswings that still get plenty of power. They accelerate, if you observe carefully, from contact upwards and mainly across.