From what I have heard it is a matter of experience. If you need to ponder about every aspect of the hand, you will need more time and attention to each table. Once you have gotten a lot of experience/study you will know what needs to be done in most situations and only have to think about the trickiest ones.
If you play chess, you wouldn't play games with 5 minutes total on the clock if you were starting out. You would be pondering how the pieces move, trying to see at least a couple moves ahead. You would need to study the standard openings. You would need a lot of experience with all kinds of different situations.
An example from cooking. An amateur cook might sit and stare at a dish cooking on the stove top. Looking repeatedly at the recipe, trying to remember the hints that the youtube cook gave, and trying to evaluate every nuance of sight, sound and smell. A line cook at a restaurant is doing 10 different dishes at the same time all night long. The line cook knows exactly how to adjust heat, timing, seasoning, et cetera, and will have it all plated in a fraction of time it took you to do one dish.
On the other hand, playing multiple tables will get you more experience. I think you have to balance getting experience quantity with experience quality. If you are reviewing hand off the table you can probably up quantity a bit, but don't go broke, and don't sacrifice too much thinking.
I have some success at 2 tables, break even around 3 and in the toilet at 4. I have recently had to admit to myself that I am still too new at this.
Then again, this is only what I have heard. I am still a novice at this, so I could be wrong.
