AMD

In Windows, access to the GPU by multiple processes is scheduled, much like access to the CPU is. Without HAGS, all of this scheduling is performed by the CPU. With HAGS, some of the scheduling in particular situations can be offloaded to the GPU.

It has nothing whatsoever to do with how work on the GPU is scheduled across the compute resources.

It will have no measurable impact on gaming performance, and it's not supposed to, if done correctly. It's only going to potentially affect performance when multiple applications are trying to use the GPU at the same time to a significant degree.

So if you're trying to play a game at the same time as you're using your GPU to render something, then HAGS might slightly reduce the overhead of both tasks sharing the GPU. You still won't actually notice a difference.