Ilya Grigorik, Script-injected “async scripts” considered harmful”:

The inline JavaScript solution has a subtle, but very important (and an often overlooked) performance gotcha: inline scripts block on CSSOM before they are executed. Why? The browser does not know what the inline block is planning to do in the script it is about to execute, and because JavaScript can access and manipulate the CSSOM, it blocks and waits until the CSS is downloaded, parsed, and the CSSOM is constructed and available. A hands-on network waterfall is worth thousands words, consider this example:

Ilya Grigorik, Script-injected “async scripts” considered harmful”:

The inline JavaScript solution has a subtle, but very important (and an often overlooked) performance gotcha: inline scripts block on CSSOM before they are executed. Why? The browser does not know what the inline block is planning to do in the script it is about to execute, and because JavaScript can access and manipulate the CSSOM, it blocks and waits until the CSS is downloaded, parsed, and the CSSOM is constructed and available. A hands-on network waterfall is worth thousands words, consider this example:
TL;DR If you use “Vary” to negotiate content, the responses will fight for the same cache space. Additionally, IE ignores “max-age” and Safari is buggy.
Every mobile browser has a feature area where it outperforms other browsers by a factor of 10–40x. The Surface outperforms the iPhone on SVG by 30x. The iPhone outperforms the Surface on DOM interaction by 10x. There is significant room to further improve purely from matching current best competitive performance.

Michael Mullany, 5 Myths About Mobile Web Performance

This blog post from Sencha has been making the rounds recently. There are a bunch of interesting little tid-bits in there and I’m glad to see these kind of research results getting published.

(An an aside: they need to do something about their graphs. It’s nice that the surrounding text provides the context but the axes are unlabeled and there are no titles. Oh well.)