I was pleasantly surprised earlier this week to discover how easy it is to lazy load images in Lightning*.
With the formatter set, images are only loaded when they are within a configurable distance of the display window. The impact, as you can see in the screenshot below, is that with Blazy enabled initial page load makes 3 image requests, load time is just over half a second, and transfers about 5K. When scrolling the page, images are loaded in as they approach the viewport.
With normal, full content loading, the page makes 19 image requests, the load time is roughly 4 seconds, and the transfer size is 600K (my source images are pretty small).
The 1.x branch of Blazy, which is what Lightning ships with, does not work when embedding Media through CKEditor nor when using the CKEditor image embed option. There is a "Blazy With Media" formatter option in Lightning. I've not played with it but it may resolve at least the first issue. The 2.x branch of Blazy also apparently supports Media oEmbed and has an input filter for inline images.
Ironically, I don't actually use Lightning on this site, a fact which I expect I'll get some grief about from the folks at work, and which, perhaps, I'll reconsider at some point. Nor do I have Blazy enabled, though I expect I'll be changing that in short order also.