var checkScrollSpeed = (function(settings){

    settings = settings || {};

    var lastPos, newPos, timer, delta, 

        delay = settings.delay || 50; // in "ms" (higher means lower fidelity )

    function clear() {

      lastPos = null;

      delta = 0;

    }

    clear();

    return function(){

      newPos = window.scrollY;

      if ( lastPos != null ){ // && newPos < maxScroll 

        delta = newPos -  lastPos;

      }

      lastPos = newPos;

      clearTimeout(timer);

      timer = setTimeout(clear, delay);

      return delta;

    };

})();

​

// listen to "scroll" event

window.onscroll = function(){

  console.log( checkScrollSpeed() );

};