The GPU doesn't need to read texels or calculate fragments that aren't visible.
So, every frame it is only copying the visible chunk of the scrollable buffer to the framebuffer. GPUs are efficient at that. More expensive rendering to the scrollable buffer, when it is either static or infrequently changing, can be as-needed instead of every frame.
That's a really well-favored set of trade offs for many cases where you need smooth scrolling.
That's a really well-favored set of trade offs for many cases where you need smooth scrolling.