Need to optimize a webpage for a course and the timeline shows this function as causing a forced sunchronous : Need to fix this piece of code in Javascript which is causing a forced synchronous layout.  Any ideas or help as to how?

    function updatePositions() {
    frame++;
    window.performance.mark("mark_start_frame");

    var items = document.querySelectorAll('.mover');
    for (var i = 0; i < items.length; i++) {
    var phase = Math.sin((document.body.scrollTop / 1250) + (i % 5));
    items[i].style.left = items[i].basicLeft + 100 * phase + 'px';
    }