if you have a quad core, it makes a massive difference. by default, wow will try to use 2 cores and that is it. all 5 wows on just 2 of the cores while the other 2 are essentially idle.

spreading them evenly helps a lot.

:!: pro-tip:
another possibility that can help performance... if you are using maximizer and have a high res monitor (ie: 1680x1050), try setting the game resolution for each region to 840x525. hopefully, this will allow wow to render just 25% of the pixels and then do a 200% zoom to the main screen.. radically lowering the performance requirement for the box