Slowly scroll down a page permanently without heavy CPU usage or laggy scrolling

I want to make a page scroll down slowly and smoothly. Well, the speed should actually adjustable. The user should also be able to scroll up manually while the script is scrolling down. First I tried this:

var autoScrollDelay = 1
var autoScrollSpeed = 1
var autoScrollTimer
function setAutoScroll(newValue) {
    autoScrollSpeed = newValue ? newValue : autoScrollSpeed
    if (autoScrollTimer) {
        clearInterval(autoScrollTimer)
    }
    if (autoScrollDelay) {
        autoScrollTimer = setInterval(function(){
            window.scrollBy(0,autoScrollSpeed)
      },autoScrollDelay)
    }
}
setAutoScroll(1) // higher number =  faster scrolling

But it was causing a very heavy CPU load and the slowest speed was too fast. And in addition to that manually scrolling up did not work properly while the code was running.

Then I tried:

var autoScrollDelay = 1
var autoScrollSpeed = 1
var autoScrollTimer
function setAutoScroll(newValue) {
    autoScrollDelay = newValue ? newValue : autoScrollDelay //using autoScrollDelay instead of autoScrollSpeed
    if (autoScrollTimer) {
        clearInterval(autoScrollTimer)
    }
    if (autoScrollDelay) {
        autoScrollTimer = setInterval(function(){
            window.scrollBy(0,autoScrollSpeed)
      },autoScrollDelay)
    }
}
setAutoScroll(200) // higher number scrolls slower

But the scrolling was not smooth when setting it too slow (e.g. 200).

Then I tried:

$("html, body").animate({
    scrollTop: $('html, body').get(0).scrollHeight, 
}, 40000, "linear");

But again the CPU load was unreasonably high and scrolling up or down manually wasn't possible this way.

Is there a better way to do this?