Well, I think it all depends on the group you have. The group I'm with was together for the better part of a year before I joined, and I've been with them over a year myself. So as a whole we get along great, joke, laugh, and I really haven't noticed too much slowdown.
I've been in games online where no one gets into character, and everyone spends more time chatting than playing, but a good DM can keep the group on track easily enough.
We have out of character conversations just as much in the online environment. We handle this in two ways:
1: we make sure everyone knows its an out of character comment. Generally we do this with the standard ((double parens)) marker.
2: We whisper to each other. OpenRPG allows you to send secret messages to one or all of the others in the room, so if you don't want to bother the others, you can just whisper and the only clue they'll have is your status will change.
Another advantage, is the chat buffer. If we get caught up in an OOC discussion and miss something important, we can scroll back up and see what it was. As long as the OOC comments dont get too out of hand, everyone can stay up with the group pretty easily.
Aside from that, things are pretty much just like a regular PnP game. (Of course, everyone is reading and typing as opposed to talking, but there are ways around that we've employed in the past.) Thanks to OOC markers and chat buffers, its easier to regain lost focus. Also, there's less clutter around the players. Their record sheets are usualy some digital version that can be sent to the DM, the dice roller means we dont need actual dice sitting near us, and a spilled drink isnt a major disaster.
On the other hand, everyone is reading and typing, instea of talking. We dont need true record sheets, nor do we get to use our dice collection. In other words, it isnt what anyone would describe as true roleplaying when held up to the typical gamer yardstick...
Play by post or PBEM? Never done those. I look into it on occasion, but it is way way too slow. I doubt Id ever be able to get into one of them.