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iMac 5K vs Mac Pro: Unigine Valley (GPU test)
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).
Rendering benchmark
Not only does the iMac have a single GPU, the rendering test here at 2560 X 1440 has to be drawn onto a 5120 X 2880 display for the iMac (vs 2560 X 1600 for the Mac Pro). And yet the iMac still beats out the dual D700 GPU and 8-core Mac Pro, which is a rather pathetic result for the Mac Pro, and very impressive for the iMac (where is the “pro” in Mac Pro with its Apple-hyped dual GPUs?).

iMac 5K 4 GHz Radeon M295X vs 2013 Mac Pro 3.3 GHz 8-core D700