If I'm just loading a web-page or playing a non-AAA game like Terarria, I don't care whether I get 300fps or 200fps. I do care if my laptop has 4h of battery life with 6W consumed, or 8h and 3W.
There's another demo if you click through on the Github repo where they start a make -j32 on the kernel in the background while Terraria is running, and (with low-power mode off) the FPS actually goes up significantly compared to EEVDF, which I would definitely not have expected from a small plug-in scheduler in userspace like this.
I'd definitely like precise benchmarks, but for a proof of concept, I think it's a very interesting demo of what's possible.
I can't imagine this would be easy to get right, and it is probably much harder to set a good default. Even so, the background compile job taking twice as long might not be a waste of time as long as I get to play Oxygen Not Included twice as long!