Having a really well-organized GM is absolutely critical. That can be a very time consuming job given how quickly things change in Shadowrun (wound modifiers, penalties, etc.). One of the other things that can bog a group down is an excess of exposition. Really from anyone, but it seems like especially from the players. You can find games where it feels like it takes three pages of posts to actually get to Matchsticks to meet the Johnson because 2 of the six runners have amazingly complex inner monologues and need to call 15 contacts before going to Matchsticks. I sort of get that. PBP games are can be a fun creative outlet and using a written format allows you to convey more information about your character than a normal tabletop experience might. And you're character is (hopefully) interesting! S/he has an interesting backstory that you want to share! If we were really honest with ourselves, part of it is showing off too. Either through system mastery or setting mastery, it's nice to flex those muscles every once in awhile. I'm totally guilty of this when I play PBP L5R games, which have all sorts of esoteric customs when it comes to etiquette and honorifics. Do I describe exactly how my daisho is tied and how deeply I bow? You bet I do! But I try to do it in a reasonably efficient fashion.
The problem is when all of this happens all at once, you end up in a game in which nothing actually happens--things just get described. Stuff that takes 10 seconds at a tabletop ("Your Vory contact tells you that Matchsticks is a pretty famous place and questions why you've never heard of it before") can run into days for a back-and-forth commcall.