Brianna:
Even in FtF it can slow things up if not handled well.
It can outright kill a game. My longest-running FtF game was a Star Wars D6 game that started in December of 1990 and ran for over a decade...players came and went, some characters got retired and replaced, and a few of the retirees got unretired...
But the game died when the GM decided he wanted to change over to D20 Saga Edition rules. He had a valid reason...the D6 rules don't scale very well to really advanced characters, especially Jedi, and after that many years, we had a LOT of advanced characters. But the D6 rules were pretty simple (still one of my favorite systems, for that very reason), and everyone knew them, so running combat was a breeze...we could have a group of nine players make it through a round of combat in a little under ten minutes, because everyone knew what they wanted to do, what dice to roll, they had a ballpark idea of how well they needed to score, etc...
It took us two sessions to convert everyone over to SE rules. The third session ended up in combat pretty quickly...and an encounter that would have taken us maybe fifteen minutes to resolve with the old rules took an entire three hour session to finish up, because of everyone needing to double-check the rules, and figure out all the extra bonuses and penalties, and this and that and...
And it was the last time that group ever got together to play that game. Totally sucked the enthusiasm out of everyone, and frustrated the GM enough that he just couldn't muster up the enthusiasm to figure out where to go from there.