core-js is the more common of the two in our index: it runs on 20,296 websites against 71 for Instant.Page, and 34 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.
Instant.Page is a JavaScript library which uses just-in-time preloading technique to make websites faster.
34 sites run both core-js and Instant.Page. That is 48% of the sites using Instant.Page, 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.