I just upgraded to IMS 2012 from MGE 2011 (both 64bit) and the performance definately has not improved. When I load my map in a basic weblayout, the basic graphics come up then the display pauses for nearly 20 seconds before appearing.
This happens with both the PHP and .Net implementations of the viewer.
MS Server 2008 R2 Standard x68
Dual 4core Xeon X5470 processors @ 3.33ghz
25 layers, most turned off at map load (I have not yet tried a map with a single layer)