This. I was recently backing up some stuff from an old Mac OS 8 machine and it was amazing how snappy a lot of things feel. But if something is not on RAM you can clearly notice it.

Drawing to the screen was also slow until accelerated 2D cards came along.

At least at the beginning not everybody had hardware that supported DMA for disk access (and those that had it, not always had it configured properly).

That meant burning CPU cycles just for doing IO.