I just scanned a 500Gb external hard drive for errors using Windows XP on my ageing single-core AMD-based PC. It took NINE AND A HALF HOURS! I imagine with a USB3 interface (or even better, Thunderbolt) and a modern PC running Windows 7 and an i3, i5 or i7 processor, the speed would be a lot faster.
The same would go for anything intensive like converting video files, WAVs into MP3s (takes me ages) and so on. Even just backing up files is a pain on an old machine like mine that does not even have SATA hard drives.
Having said that it runs the web smoothly and everyday computing tasks. If XP wasn't so capable I would have rushed to get Vista when it came out. But the truth is, even now, it still works fine. Of course I am tempted to upgrade, but there's no point until I get a whole new PC.