When the Nehalem-based Mac Pro were released, many users noticed that the single Quad Core Xeon CPU model was faster than the corresponding dual Quad Core CPU model with many applications, Adobe Photoshop included. In fact, it was a bit like if OS X was loosing time in distributing work load and collecting and interpretatin results on a larger number of cores. As reported by DigLloyd, it seems that Apple modified some layers in the system to improve multreading management with Mac OS X 10.5.7:
"I had previously written When More is Less, showing that disabling half the CPUs could dramatically speed up certain Photoshop operations on a dual-CPU Mac Pro Nehalem. Well, that gap has narrowed considerably; the disabling trick still improves matters, but only bya small amount. various Photoshop filter tes
