That's almost laughably incorrect, although unfortunately popular amongst a certain segment of the tech set. The nearly-impossible-to-use computers that did very little very slowly back in the 80s (and earlier) were not nearly as amazing as your memory is fooling you into believing. Sure, nearly-bare-metal OSes don't soak up much in the way of resources, but that only works in very narrow market segments, and computers are totally mass market now.

And in any case, you mean "before Windows" if anything, because DOS was fast as blazes (and did pretty much nothing as a consequence).