The browser keeps scripts, stylesheets, fonts and images after the first visit, so the first load carries the whole theme. Look at the first load's waterfall in a private window. In order:

  1. One bundle for the whole site. The checkout's or a form library's code loaded on the home page (Bundle size).
  2. Images at their uploaded size on the first page people land on, usually the home page and its hero (request the size you display).
  3. Every font weight preloaded, or fonts from several families (loading fonts).
  4. Third-party scripts from a tag manager, which are downloaded on the first visit and often run again on every page (the tag manager problem).

Two first-load effects are expected and pass. After every deploy, asset URLs carry the new version and are downloaded again, once. And the first request for a new image size, and the first visit to a page after its cache was cleared, wait for the image to be generated or the page to be rendered; the next visitor does not.