Get the new iMac 5K at B&H Photo and see the Lloyd’s computer gear wishlist. 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.
This test is as real world as it gets: it is my most performed job in Photoshop, one that idles further activity while it runs. The task is long-running, so it is the #1 bottleneck to my photographic workflow (preparing multi-resolution image series in a layered Photoshop document, typically aperture series or ISO series or similar).