A couple things, and first speed after the bounce:
What subtracts pace after the bounce is grit, whatever grabs the ball. No ball can ever spin nearly as as fast as it moves forward, which means that even with a massive amount of top spin, the ball still grabs the surface and loses forward motion. I can prove that mathematically but won't bother here. For anything even approaching "flat", which is closer to neutral in that a true flat shot is not spinning forward or backward, a huge amount of forward motion before the bounce is lost as the ball grabs the court - or the court grabs the ball, according to your perspective. For slice it's more obvious, which is of course why heavy slice checks as it does.
The moment you make a surface slicker, or the ball act is if it grabs less, the ball appears to lose less speed after bouncing, which converts to the perception of speed. So say back in the 90s, if you courts were slicker, less gritty, smoother (all difference words for the same thing) then balls would shoot through quicker. That's the speed part.
The next part is height of bounce, what I think is the biggest factor. The human brain is very strange in that it will "see" what it expects to see. Anything mushy/more giving/less hard is going to result in a lower bounce. Lower bounce usually means less speed, but to the player and spectator it looks the opposite because of reaction time. If ball A bounced 50% higher than ball B, ball B may actually be moving slower, but you have much less time to react, which is why low bouncing grass took and still demands the quickest adjustments and so the shortest preparation. Old grass not only bounced lower - let's forget about "speed" for a moment - so players were always lunging one way or the other making last minute adjustments to height/trajectory, and back in the Laver days, lower bounce, smaller racket heads, less power from the rackets, less spin, carved up grass, they just had to get to the net to control the point. Think of Borg, obviously not a natural volleyer, force to attack players like Mac. Borg looked like a net player in comparison to most player today because the nature of the game demanded it. On clay he usually came to the net to shake hands, like Evert.
There's even more to this. I spent months doing research, but basically speed is more about perceived speed, meaning the time players have to adjust. On modern clay, for a variety of reasons, European clay is the same gritty monster it always was, even yanking pace of Nadal's heavy topspin. The reason why Nadal's shots on clay are so "heavy" actually works in reverse. That nature of the way grit takes pace of means that Nadal's shots come through slower with topspin than on smooth surfaces, but you don't see it that way because the differential between his heaving slice, more moderate spin and heavy topspin causes a massive change in pace from shot to shot. Players dive forward to try to get a racket on slice, then next they have to jump back no to have the topspin jump out of their strike zone. Also it is possible that making balls that fluff up more, or that have a slightly different felt, will grab even more. Then a variety of changes make the ball sit up really high, so the greater spin of poly makes taller players more and more comfortable and makes players under 6 feet tall damn near strike balls over their shoulders. High bounce means a lot more time on any surface. The biggest factor today is that higher bounce.
It is what makes things look slower, because no matter how fast the ball is moving, if a player has more time between the bounce and when he has to strike the ball, that still gives him way more time. Hence Nadal successfully standing back in the Arctic and the Antarctica even on grass. People slam him for it, but it's mostly smart although extreme, given modern courts and rackets. On old grass he would not even get a racket on many balls - no time.
