Setting the scroll increment to 5px diminishes the lenticular effect; it's like skipping frames in film. Your mind interpolates the result, but it's not as smooth as scrolling by 1px and reducing the time interval to speed things up instead.

Try this one instead to see the difference. This will scroll at approximately the same rate as your example:

setInterval(function() { window.scrollTo(0, document.body.scrollTop + 1) }, 12);

I prefer it a bit slower though:

setInterval(function() { window.scrollTo(0, document.body.scrollTop + 1) }, 30);