One point made in OP is that not everyone has the latest fastest hardware. Did you check out the clip comparing code.gov rendering their SPA on a new iphone vs a lowend android device ? Millions of people are dealing with extremely subpar experiences because developers are building their website on a work-provided macbook pro on a gigabit connection to their office...
Sure, chrome lets you simulate 2G connections and all that, but last time I checked it didn't simulate the slowdown of compiling 2MB of javascript on an old phone. Could be a revealing constraint to put in the developer tools.