I think the obvious point is that whilst what photoshop does is very complex and computationally intensive, it's not new. The vast majority of the functionality

I think the obvious point is that whilst what photoshop does is very complex and computationally intensive, it's not new. The vast majority of the functionality in an average photoshop file is the same as was in the product 15 years ago. We really have reached the point of diminishing returns. And doing those tasks 15 years ago wasn't slow, so it's questionable why photoshop hasn't scaled in speed with the hardware.