core-js is the more common of the two in our index: it runs on 16,939 websites against 257 for Lozad.js, and 118 sites use both. Counts come from real scans, not vendor claims.
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
Lozad.js is a lightweight lazy-loading library that's just 535 bytes minified & gzipped.
118 sites run both core-js and Lozad.js. That is 46% of the sites using Lozad.js, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.