It's a Lot More than the Grass, and Bounce is More important than Speed
Here's what slowed down Wimbledon. Also, the balls bounce much higher, which I contend is more important than the outright speed, turning Wimbledon into almost another clay court tournament.
As Boris Becker said, "I couldn't win at Wimbledon now, it's way too slow."
1) Denser-packed dirt
2) Different, thicker grass
3) Larger (8% CSA) tennis balls (type 3 Slazengers)
4) First three meant grass inside the service box is not worn out, because no one volleys, which means serves are slowed down, but by the finals, ground strokes bounce wildly because the baseline is like cracked concrete.
BBC did a video showing two near-identical 120 MPH serves by Fed, at Wimbledon, years apart. Second one bounced nearly a foot higher and was 10 MPH slower when it reached the returner.
Result: Nadal came to the net less often in his five-set win in 08, than Borg did against Vilas at the French Open on clay, in three sets. Back then, some people ridiculed Borg for being boring, and one dimensional, because he rarely came to the net. Now, Borg would look like a raging maniac of a net rusher. This is a main reason players with one-dimensional games are doing well. Full-court play is punished.
It's also a main reason ATP matches are getting longer and longer, along with players that stall for 40-60 seconds on first serves.
There are only (2) fast-court tournaments left on the ATP, the Paris Open (which was great, lots of variety, none of this park-10 feet behind the baseline for 30 an hour, rallies), and to a lesser extend the year end ATP World Cup ( which isn't fast, but has low bounces).