If you write a spreadsheet for somebody else, you might be using a sixty six megahertz four eight six duel processor erm, spaceship, and, somebody is using a steam driven I B M A T or something, and it runs quite acceptably on your machine,a reasonable speed, and you give it to somebody else on that steam driven thing, and it takes ages to do anything, and er, you press a button and they wait half an hour  sort of thing.