Comment 617 for bug 620074

The data transfers are done by DMA where possible, but you still have to do all the housekeeping, controller management, I/O queue handling and the like. On a 32bit box there can also be a lot of memory management work involved.

Old (pre AHCI) controllers need PIO for some parts of a transfer. That is a hardware limit.

And the kernel does switch to other processes and back and forth between them when one is waiting for I/O. The gnome shell is a very large program so on any system without vast quantities of memory the shell tends to be waiting for stuff to come from disk when there is any memory pressure. Last time I looked the compositor was single threaded with all of that so Gnome 3 stalled horribly under paging. That I'm afraid is mostly a problem in Gnome 3.

Rotating disks are in relative terms very very slow. They've not materially improved in the past ten years yet memory sizes have grown vastly, processor speeds have grown likewise. They are also very bad at trying to do two things at once so writing a large file to disk tends to really slow down reading.