Smooth 60 Hz 320x240 256 color "mode-x" scrolling was for free on VGA. Just had to update the start address and panning registers. You could also have a hardware supported stationary portion.

70 Hz 320x200 mode 13 was a different matter. Had to use CPU to update it.

That might be because VGA ran at 70Hz rather than 60 in its 256 color mode.