The first release candidate (RC1) for WordPress 5.7 is scheduled to be released today. One of the features that has been announced is that iframes will be lazy-loaded by default, using the browser-level loading attribute. This will bring performance benefits similar to those that WordPress 5.5 introduced by lazy-loading images. You can read the announcement here.
The primary place iframe native lazy loading will have an impact is on posts or pages with embedded content. When you use the Embed block to include a YouTube video, this generates an iframe. Justin Tadlock did a good overview and assessment of this change on WordPress Tavern. He highlights concerns that have been raised for sites that display ads. His post is definitely a recommended read.