Lazy-loading images is one well-known win for site speed, and at present if you want to implement it in WordPress you have to choose among one of many subtly different plugin options.
It now looks like WordPress 5.4, due out in March, may include default lazy-loading for all images.
I think bringing lazy loading into WordPress core as a “by-default” behavior for images is a really cool idea. To tell the truth, I couldn’t name the most recent changes to WordPress since 5.0, but default lazy loading seems like a really great and sensible win for site speed. Lots more detail in the linked article.
Image credit: Ralph Arvesen