I thought the biggest issue is once that file is downloaded it has to be unpacked, parsed, and executed in the browser that at the same time is trying to decode

I thought the biggest issue is once that file is downloaded it has to be unpacked, parsed, and executed in the browser that at the same time is trying to decode JPEGs and reflow the layout. It's why pages take 20 seconds or more to load. Less javascript means less cycles burned executing boilerplate framework factories means faster page loading means longer battery life means you can keep that two year old phone that's not quite so fast instead of throwing it in the trash. jQuery causes global warming. (tongue-in-cheek of course, but not entirely)