Most of the pixels don't change every second though. Compositors do have damage tracking APIs, so you only need to render that which changed. Scrolling can be mostly offset transforms (browsers do that, they'd be unbearably slow otherwise).
replyThat’s not the slow part. The slow part is moving any data at all to the GPU - doesn’t super matter if it’s a megabyte or a kilobyte. And you need it there anyway, because that’s what the display is attached to.
replyNow, the situation is that your display is directly attached to a humongously overpowered beefcake of a coprocessor (the GPU), which is hyper-optimized for calculating pixel stuff, and it can do it orders of magnitude faster than you can tell it manually how to update even a single pixel.
Not using it is silly when you look at it that way.