2017 iMac 5K vs 2015 iMac 5K : Lightroom Import, Generate 1:1 Previews + Lightroom Export Full-Res JPEGs
The 2017 iMac 5K takes the speed crown for import, outrunning an 8 core 3.3 GHz Mac Pro , that is, the fastest 2013 Mac Pro that can be built for Photoshop/Lightroom, but one not available from Apple (Apple sells a 3.0 GHz 8 core).
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 four and dual GPUs versus a single GPU.
Apple has done a very nice job of making the 2017 iMac 5K a clear-cut win over the 2015 model, excepting perhaps the small I/O performance of the SSD.
See the MPG review of the 2017 iMac 5K .