What densi77 said is indeed an issue which we are working on. You may have noticed that on Saturday, while we didn't even hit the cap, we had much worse performance than on Sunday. We worked through the night to make sure that wont happen again on Sunday, and implemented several optimizations and improvements on Sunday morning. Our team managed to literally half the server diff, which already made it much, much more enjoyable to play.
The issue is that Icecrown and Lordaeron thanks to their different rates and setup are quite different in terms of player density in cities, which is a major performance drop for Icecrown. We can see that Dalaran, Orgrimmar and Stormwind are often so packed, even the best PCs have problems with FPS. Thanks to our completely new visibility system we could reduce the visibility range in those cities (at least temporarily) to improve the FPS for players. Our core by now is already extremely optimized and tweaked, otherwise we wouldn't even be able to hold 2.000 players, but our focus now is to continue optimizations and step-by-step improve performance. However, at this point there is no magical solution that will improve performance by huge percentages. A lot of smaller modifications will need to be made to see an impact.