How to make snap scrolling smooth

i currently use: 

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/scrollify/1.0.19/jquery.scrollify.min.js"></script>

<script>
    $(function() {
        $.scrollify({
            section : "article .page-section",
            scrollSpeed: 1800
        });
    });
</script>

it works but it's very glitchy,

The site is: Nikic Costruzioni By Goran & Boban Nikic 

If someone knows how to resolve the issue please let me know 

Thanks in advance 

Try this to Custom CSS then check again

body.homepage article section {
    min-height: unset !important;
    height: 100vh !important;
}