I don't think that the spinning speed has anything to do with scanning speed or size of the scanned file.
The icon spins when there's anything being scanned - but the spinning speed is constant all the time (I believe it's ruled by a counter - if there's any scan in progress, the icon is rotated by one "piece" every 0.2 second, for example (I just made this number up, don't know what's the exact value).
But yes, if the system is too loaded with the scan itself, it might not be able to redraw the icon quickly enough, so it looks like it doesn't turn smoothly.