Starting MediaWiki 1.34 there is a config variable to enable native lazy loading: https://www.mediawiki.org/wiki/Manual:$wgNativeImageLazyLoading

Native lazy loading is handled by browsers, the only thing to do is adding the attribute: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img#attr-loading

User avatars are images most commonly used with SocialProfile, and we can have a significant amount of them (especially if you use the Comments extension). I think it's great to enable lazy loading on avatars when wiki is configured to use it.