Endurance and burnout are big factors in this issue.

While 7-tabling on one screen I did suffer some burnout.

My level of thinking decreased and autopilot took over whenever I had 3 or more hands going at the same time.

Also you need to do all them quick default decissions and this leads to burnout I think.

Another thing to consider is how much your learning curve suffer form multitabling..?

The good things in multitabling: You can play a lot of hands on short time thus you can get a good sample size in little time. This is good for confidence to you know theres a very good chance that you are a winner in a particular game.

If you can do it, then it makes wonders for building a bankroll