It actually works both ways for me.
If I'm playing with a quick moving group, that is focused on the round at hand, I play just about as well as I do when I play solo rounds. Sometimes I might play a little better if my competitive nature kicks in, even during a casual round. Close competition can push me to improve.
If I'm playing with a slow moving group, that isn't focused on the round at hand, I can play worse than my solo rounds unless I really bare down and focus on my own shots.
I'm not a guy who needs silence to play, and I can focus very well in the face of mild/moderate distractions. But when players in my group are ultra slow at making their disc selection, taking 26 practice putting motions, are searching for lost discs in-between holes, talk more than play, etc., I enjoy the game much less, and my play can suffer.
I've only played one sanctioned tourney and some ace races. I had no problem focusing and playing well in those cases. I guess the groups I got paired with worked out well.
Disclaimer:
If any of my slow playing friends read this, I still really like you guys. It would just be more fun for me to play if we picked up the pace just a bit.