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2017 iMac 5K: Lightroom Import and Export
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).
A note on the 8-core 3.3 GHz Mac Pro
These tests use a 2013 Mac Pro 3.3 GHz 8-core, the fastest Mac Pro available, and one that Apple does not sell—it needs to be an upgraded CPU. 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.
Lightroom import with 1:1 previews
Import without moving 156 high resolution raw files with 1:1 previews.
The 2017 iMac 5K handily beats its 2015 predecessor (14% slower), and takes the the speed crown, outrunning an 8 core 3.3 GHz Mac Pro.
The 14% speed difference between the 2015 iMac 5K and 2017 iMac 5K is just one more good thing about the 2017 model; it is one of the most impressive Mac revisions in many a year for the consistent speed gains versus the prior model.
Which demonstrates just how poorly Adobe uses CPU cores: twice as many cores, each running somewhat more slowly but it is still 8 cores versus 4 and dual GPUs versus a single GPU.
Lightroom export full-res high quality JPEG
The 8-core 3.3 GHz Mac Pro shows a decisive advantage, with the 2017 iMac 5K taking 19% longer.
The 2015 iMac 5K takes 15% longer than the 2017 iMac 5K.