This will have a positive effect on your PageSpeed score, especially for the mobile test.
Simply check the difference with and without this add-on.
Loading image lazy means your browser will only download images when needed.
It's one of the PageSpeed recommendations:
Defer offscreen images
Consider lazy-loading offscreen and hidden images after all critical resources have finished loading to lower time to interactive.
Google Chrome does this in an intelligent way and decides based on desktop/mobile usage and connection speed when to load images.
There will not be any visual impact, so don't worry about images fading in or other annoying effects!
Also, this add-on uses the native 'loading' tag which was introduced recently, so no extra java libraries needed.