Federer would have 20 slams if courts played at late 90s speeds.
Nadal would have 2-4 less slams.
Djoko would be about the same.
Grit on the surface is one key factor as others said above. I also think the amount of cushion and how soft the cushion is under the court has a lot to do with it too. Most public and club courts are not soft. But, Australian Open supposedly had a softer give to the surface which slowed it down a bit. The AO looks almost like clay court tennis on TV.
I would like to see courts sped up just a little bit. Racket technology and poly strings have giving the returner and ground stroker too big an advantage. It is time to give the server and net rusher some advantage. I think grass and a fast hard court are the best tourneys to watch on ATP. I like seeing players come to the net and hit a variety of top and slice from the baseline instead of a battle of retrieving and topspin bashing.