I think that (one of the things that) Mustard is trying to say is that players today (on average) impart much more top-spin than in previous eras. And because of that much top-spin the balls travel slower (take more time to get to the other baseline) than with a very flat hit shot. That alone can make the impression that the conditions are slower than they actually are.
And that is true.
But apart from that, I really think that many outdoor hardcourts (AusOpen, US OPEN, and many others) have been slowed down noticeably, and they are using slower balls as well.
You (Mustard) say that todays players average shots have more power than earlier, and that is true, but probably the main reason is that in todays conditions the balls sits up in a way that did not happen on faster former conditions.
When I watch Agassi, Martin, Chang playing in the US OPEN in the 90s, their average shots may have been slower than today (the court and balls were faster and after the bounce the balls did not sit up as today, it was more difficult to play on those former faster conditions), but you could see that when they had an easy ball, a ball that sit up after the bounce, they smack a flat shot that looked WAY FASTER than the average winner shots you see today. Todd Martin flat attacking shots, even Chang when he decided to smack a flat winner off of an easy ball, were absolutely impossible to retrieve, whereas today you see that it is much more difficult to hit a shot like that (because the balls are slower and they lose much more speed after the bounce today).
So there are many different things that contribute to what all of us are watching: slower hard courts, slower balls, and much more top-spin on average.
So, have they slowed down hard courts and balls? For me it is clearly: YES (apart from stopping using carpet and slowing down of Wimbledon grass and balls there too).
Do the type of strokes influence the perception of speed? Yes, of course. I remember when Todd Martin won the Barcelona Open one year in the 90s against Alberto Berasategui in the final, that suddenly that slow clay of Barcelona looked WAY FASTER only because the powerful flat shots of Todd Martin were absolutely impossible to retrieve for Alberto Berasategui.
Is it this thing what you are trying to say, Mustard?
But apart from that, they have slowed down some hard courts and balls noticeably.