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iMac 5K vs Mac Pro: Image Scale (PhotoZoom Pro)
iMac 5K 4.0 GHz, 32GB, 1TB SSD, AMD Radeon R9 M295X 4GB GDDR5 -versus-
2013 Mac Pro 3.3 GHz 8-core, 64GB, 1TB SSD, D700
The two machines tested here are the fastest possible models for this type of work (as of late 2014).
PhotoZoom Pro 5 (image scaling)
Photozoom Pro has GPU support including an optimization setup which is claimed to determine best GPU performance.
While the Mac Pro with its dual D700 GPUs and 8 cores wins here, the iMac with its four CPU cores and single GPU is not far behind.