Perhaps it depends on the system (hardware and software) as to how much benefit you get ?
That's what I might be thinking, too. A lot of people say they hardly notice a speedup with turning the graphics processor usage on - but I notice a pretty hefty speedup here, for the things it's supposed to speed up (and also slow down). So I usually keep it turned on.
I strictly use a ~2012 AMD 4-core Win10 laptop with the onboard graphics. Although I read, at the time, AMD's onboard graphics were supposed to be faster than Intel. But I'm thinking it could have something to do with the shared memory it uses for the graphics.. Might be less 'overhead', if that's the word I'm thinking of.
I know the image I was on had profile corrections and upright turned on, cause that would be a normal culprit I would check. I think I must have done roughly about 70 spot removals on that one. I haven't used LR much since then, cause I'm into other things right now - but I keep meaning to check further into any seeming-to-be speedups. Although I do remember pulling up an image with many brushings, and
that was still slow as molasses (didn't check spotting) - so for all I know maybe it's primarily spotting on certain systems, as far as local adjustments go.