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iMac 5K (Late 2015) vs Mac Pro: Convert 50-Megapixel RAW Files to TIFF (Iridient Developer)
Get iMac 5K at B&H Photo and see also MPG’s computer gear wishlist.
See also diglloyd-recommended performance packages for iMac at OWC.
See also iMac 5K: Building a High-Performance Configuration.
The two machines tested here are the fastest possible CPUs as of late 2015.
Canon 50-megapixel raw file converson to TIF
This test is a real-world task using Iridient Developer, which is efficiently multi-threaded.
While the 3.3 GHz 8-core Mac Pro wins here, the iMac takes only 17% longer in spite of having only 4 CPU cores and one GPU, which is a disappointing performance for this very high end Mac Pro (8 CPU cores and dual GPUs!).

late 2015 iMac 5K 4.0 GHz M395X vs 2013 Mac Pro