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Mac Pro Westmere Performance with PhotoZoom Pro
See test machine configuration.
January 2014 update
Here we see perfect scalability matching the theoretical limit (within the bounds of measurement error). Compare to the severely impaired scalability of Adobe Photoshop CC.
Taking the 4-core time as a baseline time and scaling it to best-possible 2/6/8/10/12 core times (e.g., 12 cores should take 1/3 of the time as 4 cores), observe that the “Extrapolate” line matches the measured results perfectly. In short, PhotoZoom Pro offers perfect scalability.
Praise is warranted here as few developers have the skill to accomplish this feat on 12 cores. It is a rare accomplishment in software for OS X*.
Click for a larger graph.
* If Photoshop CC could scale this well, a 3.33 GHz Mac Pro would be equivalent to 8.25 GHz, which would be about equal to the speed gains in 7 years of Mac Pro CPU advances. Software makes all the difference.

2010 results
Here we have an example of outstanding scalability: the run-time is proportional to the number of cores. Very few programs can do so well in using all the available cores.
If only more programs exploited all the CPU cores this well! Even the 2.4GHz model did well here, because all eight of its slower 2.4GHz cores are used. But it’s still slower than the 3.33GHz 6-core.
The 12-core model trounces everything else, with the 8-core 2009 Mac Pro following next, then the hexacore Mac Pro. Cores rule here.

CPU core usage with PhotoZoom Pro 3.1
See Monitoring system performance with Activity Monitor.
Outstanding! This is what you’d like to see with every program— full CPU core usage that scales efficiently ( it is possible to use all cores in an inefficient way).
It is a rare and beautiful thing. PhotoZoom Pro scales perfectly—6 cores are 50% faster than 4 cores.

2010 Mac Pro 6-core @ 3.33GHz