スクロールに連動してで滑らかにオブジェクトを移動しつつ、スクロールのイージングをコントロールする。

const scrollSlow = () => {

  let $window = $(window);
  let scrollTime = 3;
  let scrollDistance = 6;

  $window.on("mousewheel DOMMouseScroll", function(e) {

    e.preventDefault();

    let delta = e.originalEvent.wheelDelta || e.originalEvent.detail * 40 * -1;
    let scrollTop = $window.scrollTop();
    let finalScroll = scrollTop - delta * scrollDistance;

    TweenMax.to($window, scrollTime, {
      scrollTo : { y: finalScroll, autoKill:true },
      ease: Power1.easeOut,
      overwrite: 5
    });

  });

}
scrollSlow();

const scrollAnim = () => {

  let delta = 5;
  let speed = 1.5;

  $(window).scroll(() => {

    let elementScroll = $('[data-parallax]');
    let scroll = $(window).scrollTop();
    scrollHandler(scroll);

    function scrollHandler(scroll) {

      $(elementScroll).each(function() {

        let el = $(this);
        let ratio = el.attr('data-parallax');
        let offsetTop = el.offset().top - scroll;
        let translateY = offsetTop * ratio / delta * -1;

        TweenMax.to(el, speed, {
          y: translateY
        });

      });

    }

  });

};
scrollAnim();