Plugin enabled in page which can only scroll vertically.
In most situations, page can only scroll vertically. At this time, only check images's vertical position will improve performance.
This example page can scroll both vertically and horizontally, we set vertical_only to true, so it only check images's vertical position to decide whether appear or not. Shift-reload to test again. Compare this to page where plugin is disabled or wide page.
You can only set vertical_only to true when page can only scroll vertically (not like this example).
$("img.lazy").lazyload({ vertical_only: true });
<img class="lazy" data-original="img/example.jpg" width="765" height="574">