On a 3090, it gave me a 720p gen in 5 minutes vs 45-50 minutes with it off. At 480p, the visual quality drop is noticeable compared to a normal gen (more aliasing, for one), but on 720p, it looks pretty damn close to a normal gen.
The motion does take a hit. I don't know how much, but of the couple of gens I've done, I've noticed they're a little more stiff than a normal gen. I'd have to do side by sides to see how much though.
Still... pretty fucking impressive, all round.