There's nothing more frustrating than when I'm trying to read through a page and my spot keeps getting pushed up and up because of dynamically loading elements.
That being said, lazy loading does at least some of the time work "invisibly" with images getting loaded before they would become visible. It's up to the browser to decide how aggressive it wants to be about this.
It used to not work for responsive images. We had to do tricks like pseudo elements with padding set to a percent value: https://css-tricks.com/preventing-content-reflow-from-lazy-l...
https://jaeh.at works fine when reloading, no layout changes, and the layout changed slightly before i fixed it using width & height.
https://noncon.org does not (and i had no time to fix, might be because width of images is not set, might be something with the css position/float layout, we had about a week to setup the whole conference infrastructure, including the streaming server, layout flashing was my last concern at the time and i kept myself too busy since then to fix something so trivial...)
sorry for the personal links, just examples i knew from the top of my head because i built them ;)