I have a GPU load indicator in my menubar. Admittedly, the reason I have one there is because I’m probably asking it to spew pixels onto more screen real estate than the designers have in mind, so I do accept that the simple answer to “what the hell is swamping my GPU?” is “you’re running a ridiculous number of displays, you idiot”.
But the computer only gets sludgy when the load indicator shows it’s saturated, and most of the time it isn’t like that and hums along just fine.
Now and then something comes along and chokes it pretty bad, though, and if I knew which tasks and situations were doing that, maybe I could do something about it. My menu widget is iStat Menus and it doesn’t give me a breakdown of what processes are responsible. Activity Monitor, meanwhile, tells me both too much and too little (it doesn’t have a GPU tab and it lists everything that’s running, including arcane core operating system stuff).
I guess what would be most useful would be something that keeps a log of the load on the GPU, if it were somewhat intelligible to me.