It’s just the UI.
Steam Deck uses React and the render sometimes are indeed slow. Especially if you’re in the middle of a game and popping up the steam menu.
It has nothing to do with the performance, just the unfortunate side effect of how render thread blocking works.
The apu does upgraded, but not in the way you think. It get node shrink which means it can do the same performance at lower power consumption.
By their own claim valve said that they can squish extra 50% battery life from only 25% batteries capacity increase.