There are studies regarding how human brains and multi-tasking work (actually don't work very well). These studies largely indicate that efficiency and performance are negatively impacted when multi-tasking. Unlike computers, human brains just aren't built for multi-threading.
Playing multiple tables is not a computer definition of executing more than one task simultaneously; but instead, changing tasks with frequency(x). First one focus on this table, then that table, then this table. At question is the performance cost?!?
A recommendation is to establish a baseline for your acceptable performance at a single table. Do not set the expectation too high... your brain can't be expected to perform at the same level in which it would with a one table focus. Add additional tables until such time that your performance degrades below what you, as a responsible manager of your bankroll, are willing to accept.