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iMac 5K (Late 2015) vs Mac Pro: diglloyd Photoshop Benchmarks
Related: 2015 iMac 5K, 4K and 5K display, GPU, iMac, iMac 5K, Mac Pro, Macs, memory, Photoshop, SSD, video
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.
Save money on 32GB or 64GB OWC memory (about $400 savings vs Apple!)
MPG recommends this iMac 5K: 27" Retina 5K Display, 4.0 GHz Intel Core i7 (Skylake), 1TB Flash, 8GB memory, AMD Radeon R9 M395X GPU (4GB GDDR5) + 32GB or 64GB OWC memory.
The machines tested here are the fastest possible models for this type of work. The iMac 5K as tested here edges into low-end Mac Pro pricing territory, at around $4K as configured with 64GB OWC memory.
A note on the 8-core 3.3 GHz Mac Pro
Apple offers a 3.0 GHz 8 core (slower than 3.3 GHz), and it would be a bit slower than the 3.3 GHz 8-core Mac Pro tested here. However, since Photoshop generally utilizes fewer than 8 cores (or even 6), it is likely that the 3.5 GHz 6-core Mac Pro would perform similarly to the 8-core Mac Pro tested here, since the 3.3 GHz 8 core and 3.5 GHz 6-core CPUs have nearly identical turbo boost behaviors.
diglloydSpeed1
The diglloyd Speed1 benchmark uses a mix of the most commonly used Photoshop operations with a file size that allows everything to stay in memory. Hence it accurately represents what one might expect in everyday use of Adobe Photoshop CC. Specialty operations such as Liquify and other GPU-intensive tasks are just that—specialty, and are not included in this suite.
As shown, the differences are negligible between configurations and within the margin of error: identical performance. Given the dual D700 GPUs in the Mac Pro, this is an impressive showing for the iMac with its M395X GPU.

late 2015 iMac 5K 4.0 GHz, 1TB SSD, M395X vs others
diglloydMedium
The diglloyd Medium benchmark uses a mix of the core Photoshop operations with a moderately larger file size that exceeds what most users are likely to use, taking about 15GB of memory usage in Photoshop.
The 3.3 GHz 8-core Mac Pro outperforms a maxed-out iMac, but only modestly. The use of 32GB vs 64GB makes no difference for this test, which comfortably fits into 32GB.

late 2015 iMac 5K 4.0 GHz, 1TB SSD, M395X vs others
diglloydHuge
The diglloyd Huge benchmark uses a mix of the core Photoshop operations with a file size that requires about 56GB of memory usage in Photoshop, far beyond what most users are likely to encounter.
With only 32GB of memory the iMac 5K lags badly (32GB is the maximum Apple offers, this test uses the OWC 64GB memory kit).
The iMac 5K with 64GB actually outperforms a 3.3 GHz 8-core Mac Pro. Most likely this is due to its much faster flash drive because this test does not fit entirely into available memory (so the flash drive has some involvement).

late 2015 iMac 5K 4.0 GHz, 1TB SSD, M395X vs others