One of the more frustrating things about Photoshop is how many operations run single-threaded, or only slightly better than single threaded. This graph shows a multi-minute job that I needed to run today many times—on the fastest Mac Pro that can be built (8 core 3.3 GHz CPU)!
Observe that CPU usage is about 130%, meaning that one CPU core of eight real CPU cores is used (800%). Put another way, 84% of the computing power goes unused, and the GPUs are useless for this task also. This is why a 4-core anything often performs as well as a far more expensive Mac Pro. Still, there are cases where 6 or 8 cores do get used; it’s all a question of which tasks for how long during the workday.
The core issue remains the same today as 10 or 15 years ago: software algorithms. There are plenty of places where Photoshop runs single-threaded where it need not. It’s a matter of software architecture and until and unless Adobe sees fit to advance in some areas, the fastest CPU speed wins for most all tasks.