There isn't much that can make an in-flight connection seem quick, but we've tried with our PWA (https://usebx.com/app) and to a certain extent, succeeded.
Ok, we do assume that you have visited our webpage at least once in an area of strong signal, however, once you've done that, all the JS is cached and all we transmit back and forth are lightweight, gzipped json payloads, often less than a kilobyte.
I use my own PWA regularly, and found it actually quite usable in-flight.
I think this is a big issue with such JavaScript and load heavy web-sites & apps. They forget that they have users without 1GB connections and who can't afford, weather financially or location based, the download the 500MBs of data required to load their web-site. That and hiding CSS inside of JS feels like a sneaky tactic to keep people from editing it? Or does it actually create the CSS on the fly? I wasn't quite sure after reading the article.
Slow internet connections are a reason to think about the experience when loading JS is slow and/or failed. It's not just 'poor people' who have slow internet - 'rich people' on inflight wifi or on the internet on the train can result in things not loading.