From recent bundle audits of e-commerce sites built with React, less than 20% of the code was actual custom site code and the remaining 80% was from dependencies.
If the bundle size is 550kb min/gzipped, that's 110kb of app code, and 440kb of dependencies. Some of the largest dependencies in a recent audit were: moment (60kb), swiper (32kb), lodash (24kb), react-select (26kb), raven.js (12kb),
polyfills (25kb), mobile-detect (15kb), and then a bunch of smaller dependencies that made up the remaining 300kb.
Using this performance budget calculator you can quickly see how each 25kb of JS adds ~.3s to your TTI on a mobile phone: