I already posted in another similar thread the results I've got from a video frame-by-frame analysis comparing AO 2000 final (Agassi vs. Kafelnikov) and this year R16 match Djokovic vs. Wawrinka, which showed that the difference in deceleration by the surface between 2000 and 2013 is only marginal. But the players abilities and tactics have significantly improved by now.
Makes sense. They measured the friction levels when they changed surfaces, Rebound Ace was 35, Plexi was the same or a few points higher (putting both in a medium category, which is 35-39). I think this was using the old surface speed measurement system, so not comparable to the current which is a little more complex.
There may be year to year variations of course, eg Federer was told they'd sped it up for this year, and about a dozen other players said it was quite a bit quicker.
Aus Open balls have got quite a crap ton slower though, as have pretty much all hard court balls. That's not a sudden thing though, it's been happening for 8 or 9 years.