When I used to play poker regularly (i.e. put $1,000's in other people's pockets!) it used to really P me off because I knew players were multi-tabling and slowing down the games to a snails pace.
I mean, if you went to a real B&M casino you wouldn't be able to play on more than one table at a time would you?
If I could stop people doing it online I would, but that's impossible of course.
I would have thought you ought to be able to get at least 6 open windows, especially if you play sites where you can re-size them to anything you want (like PokerStars) - then you can piss-off 54 other people all at once!
I've got up to around 20 tables of full ring on Full Tilt.
Really isn't too difficult if you stack them correctly and pay attention.
My play did suffer slightly, I find my optimal level is 6 tables tiled so I can pay full attention to each table at once.
If you're working through the deposit bonus, and are a competent player, heavily multi-tabling will help you race through the bonus.