I wouldnt say that Federer is a fast starter, not at all. He usually plays ridiculously bad in the first games of the first matches, he can get to 4-4 with players such as Young or Phau and not get a single break point. Nadal is a slow starter, too. Djokovic isnt too fast either, getting broken by Lorenzi in the first game is humiliating. Dolgopolov is a terrible starter as well. Wouldnt call Murray too fast, either.
Fast starters are usually the guys who serve well. People like Raonic, Berdych, Tsonga tend to start well and not get broken in the beginning.
Maybe now for some matches, but Federer especially in his prime was/is generally a fast starter, I mean he has a well enough serve.. Berdych has a rather inconsistent/bad serve, not sure what you're thinking. Hell, when Federer plays Rafa too even these days, he normally comes out of the gates running.
Nadal a slow starter? I've lost count of how many times he breaks his opponent in the very first game, don't think anyone does it as often as Rafa.
Sure against lower ranked opponents perhaps, but I've seen plenty of times over the years especially at the later stages of events where Rafa is just a slow starter. The finals of French Open 2006, Wimbledon 2006, Wimbledon 2007 are a few examples. Plenty of other times he has played Federer like at this year's Australian Open and Indian Wells, he starts out pretty slow. Obviously it also has something do with Rafa's opponent, but IMO he is more a slow starter than a fast one.