fakescroll.js

page.evaluate(function() {
  var current = 0, delta = 800, total = document.height - delta;
  window.scrollTo(0, current);

  function fakeScroll() {
      if(current < total) {
          current = current + delta;
          window.scrollTo(0, current);

          window.setTimeout(fakeScroll, 100);
      }
      else {
          //window.scrollTo(0, 0);
      }
  }

  fakeScroll()        
});