This is a talk by Ilya Grigorik of Google’s Make The Web Fast team, presenting a lot of hard data about the negative effects of slow loading web pages, the mechanics of downloading and rendering pages, which parts of those mechanics can cause significant delays (particularly on mobile networks), and practical techniques to mitigate these.

It’s somewhat focussed on US carriers and 3G/4G networks in places, but probably relevant to anyone working on a web app and particularly on a web app with a significant mobile-user market.