Mac Performance Guide has tested the latest iMacs against the fastest Mac Pros available and released its benchmark test results. They performed a pure speed test that did not factor in disk I/O, so these results depend on raw CPU speed only. The 3.4 GHz iMacs came out faster than the current 8 and 12 core Mac Pros, and even faster than the MacBook Pro with Retina Display.
The latest 2012 iMacs from Apple provide great improvements over the previous generation. Benefits include better cooling, better speakers, Fusion drive, USB 3.0, and great CPU/GPU options.
The results are not unexpected; Photoshop CS6 does not use even two CPU cores well so the latest generation Intel Core i7 running at 3.4 GHz easily outpaces a 3-year-old Xeon running at a slower clock speed.
Some of the speed difference might be Photoshop itself: the slow 12-core performance is not so much a criticism of the 12-core Mac Pro as of theperformance bugs with Photoshop CS6, which suffers from internal implementation inefficiencies (performance bugs) on systems with more than 6 CPU cores.
When Photoshop runs with only 4 cores (8 virtual cores) instead of 6 (12) or 12 (24) cores, it is simply more efficient (less overhead, fewer bottlenecks).
Until Adobe changes how Photoshop uses CPU cores, this probably won’t change any time soon – even if Apple does update the Mac Pros to use the latest processors. However, this is to not to say that Apple shouldn’t update its Mac Pro line. The iMac still has limitations that prevent its adoption in certain applications. For tasks where an application uses six or more CPU cores efficiently and/or use more than ~24GB of memory, the 6-core and 12-core Mac Pro will be stronger performers.