my guess is... not at all these days.
I mean, it made a difference when hard drives where slow and computer bus speeds comparatively so. not so much today. even a 10 year old computer gets pretty decent transfer rates.
besides, SSDs are dirt cheap today and the whole argument becomes a moot point.
you are going to find 30 things slowing down your mame experience and harddrive speed is going to be at the bottom of that list.