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2017 iMac 5K: Real World Photoshop: Make Multi-Res Image Series
MPG recommends 64GB OWC memory (big savings!) in: 27" Retina 5K Display, 4.2 GHz Intel Core i7 (Skylake), 1TB Flash, 8GB memory, AMD Radeon Pro 580 Graphics Card (8GB).
This test is as real world as it gets: it is MPG Lloyd’s most performed job, and one that idles further activity while it runs*. The task is long-running (minutes), so it is the #1 bottleneck to your Lloyd’s photographic workflow, that is, preparing multi-resolution image series in a layered Photoshop document, typically aperture series or ISO series or similar.
This scripted task creates a range of output JPEGs of varying resolutions for each layer, typically to 8 layers, with a dozen image sizes for each of those layers, from 432 pixels to 6048 pixels.
Photoshop is not good at using more than one or two CPU cores for general tasks, and this shows: the 4.2 GHz CPU of the 2017 iMac 5K trounces the fastest Mac Pro you can (cannot) buy: the 3.3 GHz 8-core Mac Pro. Here we have an about $4000 iMac 5K trouncing an ~$8K Mac Pro. For the Mac Pro user (Lloyd), that hurts! Every other Mac Pro would be even slower than the MPG custom 3.3 GHz 8-core, which makes the iMac 5K look even better.
The 2015 iMac 5K lags the newer 2017 model by only 5.4%, corresponding almost exactly to the 5% CPU clock speed difference (4.2 vs 4.0 GHz and 4.5 vs 4.2 GHz turbo boost). Clock speed thus seems to be the only factor involved.
* Photoshop is badly implemented for scripted jobs, being unable to run in the background without glitches, and constantly forcing itself to the foreground, making it impossible to do anything else. Running a script in Photoshop essentially idles the machine for other uses while the job runs.