A bow is essentially a spring.
On occasion, I've had to help customers when they wanted to launch an object at x feet per second with a spring. The design of the spring significantly influences how fast it releases energy. The release speed is basically proportional to stress, for a given metal, but it also varies by type of metal. For example, titanium springs release energy faster than steel springs, because they store more energy per pound.
In a bow you've got various composite materials that flex, combined with a sophisticated mechanism (the pulleys and string) that magnify the release velocity.
Archery is very competitive, so manufacturers are constantly trying to refine their designs, not just for marketing hype, but also to gain speed & accuracy. I would guess that "smoothness" and the "back wall" also influence shootability, which is somewhat subjective.
