How can I make the webpage slow down autoscrolling on a button press?

Question

As the title says - I'm looking to add some auto-scrolling on a specific webpage but the script that I have at the moment stops it completely from scrolling, which doesn't look visually appealing during presentations IMHO.

Is there a possibility to slow down instead of harshly stopping the auto-scrolling of the webpage on a specific keystroke?

This is the JS that I tried.

let scrollerID;
let paused = true;
let speed = 2; // 1 - Fast | 2 - Medium | 3 - Slow
let interval = speed * 5;

function startScroll(){
    let id = setInterval(function() {
        window.scrollBy(0, 2);
        if ((window.innerHeight + window.scrollY) >= document.body.offsetHeight) {
            // Reached end of page
            stopScroll();
        }
    }, interval);
    return id;
}

function stopScroll() {
    clearInterval(scrollerID);
}

document.body.addEventListener('keypress', function (event)
{
    if (event.which == 13 || event.keyCode == 13) {
        // It's the 'Enter' key
        if(paused == true) {
            scrollerID = startScroll();
            paused = false;
        }
        else {
            stopScroll();
            paused = true;
        }
    }
}, true);

Thanks in advance!