With a pushrod engine the cam is in the block, followed by a lifter, the pushrod, a rocker arm and then the valve. This valve train is much heavier and less rigid than an overhead cam which uses a cam follower or rocker arm and then the valve. The lighter, more rigid overhead cam valve train therefore generally allows higher revs and more radical grinds. The result is greater horsepower at greater revs = more speed. That's it, pretty much simplified.
Pete