We are increasingly working with Photoshop HR files with a file size of around 15GB and with many layers. Our current 2017 iMacs with 16GB ram are no longer sufficient. But it is quite difficult to determine whether a faster graphics card, a faster processor or more RAM would be more useful to us. Which part is more important when working with these large files? Because of the budget, I'd prefer to stay on an iMac rather than move to a Mac Pro.
Could someone from experience tell us more about this? What hardware do you use?
What you need to consider is that Photoshop is mostly bandwidth-limited, not CPU limited. It handles a massive amount of data and this is what slows most people down. In addition, lots of functionality is handled by the GPU these days.
The single most important thing when working with large files is the scratch disk. Make sure you have plenty of space on a fast drive (NVMe). Without enough scratch disk space, Photoshop grinds to a halt.
For these file sizes, you should have at least 1 TB free space available.
As for RAM, there is no such thing as "enough" anyway. The scratch disk will carry the heavy load, while RAM is more of a cache to keep the flow going. Personally I think 64 should be enough, but go to 128 if you feel like it.
The GPU is becoming very important for overall functionality and performance, but on Mac this is all integrated in the M1 architecture and I'm not up to speed on that.