Suspension parts distract or take away from the riders efforts. In other words...it soaks up energy both from the road & the rider. Also, a good - I mean really good full suspension bike starts at $1,000 or more. Those cheap ones in discount stores literally fall apart. One of them fell apart while a neighbor of mine was ON IT!
What other things effect the speed of a bike? Weight, width of the tires, tread pattern of the tires, air pressure in the tires, aerodynamics, the riders ability, gear ratio, etc.
Hard tails are cheaper and some people don't need the full suspension. Hard tails can go faster than full suspension because the suspension below the seat (mid?) slows you down.