All of this complexity (with "browsers also differ quite a bit in the importance they assign to different types of resources and loading methods," etc.) suggest

All of this complexity (with "browsers also differ quite a bit in the importance they assign to different types of resources and loading methods," etc.) suggests one take-home message. It is very expensive to architect your web services with the optimal combination of performance across browsers. Unless you have a huge budget, your most practical option is to be performant only for the browser with the largest market share. (Or hope that you can outsource to someone who understands all of this and yet remains affordable.)