imo, people are using the term "fast" in very different ways. Fast can mean the ball speed itself, i.e. the velocity of the ball after leaving the rubber surface. It can also mean the time the ball makes contact with the rubber, i.e. short contact time equals fast, or dwell time if you would prefer the term. So, in terms of fast in the "contact time" aspect, I think stretching makes the rubber "faster", since stretching -> harder surface -> less contact time. But whether the ball travels faster is a whole other story, and I don't have an concrete answer to that, but my instinct from my years of taking engineering courses in college and grad school tells me that it depends... on a whole lot of variables.
The spring equation is F=-k*x, where k is the spring constant or coefficient, which basically is the stiffness of the spring if the material isn't changed, and x is the distance the spring is compressed. So if you use the same exact stroke (F is kept the same) to play a rubber that's harder, it means the rubber will compress less (k increases, so x decreases). And the spring potential energy is dictated by these as well P.E.=F*x, so with less compression comes less potential energy, and when that energy is turned into kinetic energy, it meaning the ball will travel at a lower speed K.E.=(1/2)*m*v^2.
This is however, not the same as saying harder rubbers (whether by design or stretched) results in lower speed. The ball will only travel at lower speeds when the stroke is exactly the same as before, but with harder rubber you can apply more force to the ball without bottoming out the rubber. And when more force is applied you can still achieve the same compression on the harder rubber, so the energy store will be higher. This is to say that the potential of the potential energy is higher with harder rubbers, or in plain English, the speed ceiling is higher on harder rubbers.
That's the rubber part, now we also have the relationship among force, speed, mass and time, F = m*(v/t) , which if you adjust the sides a bit becomes F/m*t=v, which basically is saying if the force (stroke) and mass (ball) is kept the same, when the force has less time to work on the ball, it means the ball will travel at lower speeds, duh. All of this makes sense if you remember that, tacky Chinese rubbers are hard, because harder means less contact time, where the tackiness comes in to increase contact time.
Phew, so rusty on these stuff, haven't touched any of these in years lol