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.