Most of the code and metrics are now outdated but https://wiki.mozilla.org/Firefox_OS/Performance will have some historical perspective, http://eideticker.mozilla.org/#/android/lg-g2x/mozilla-centr... was our test for comparison between android and fxos.

When we started it was an open question whether we could build a fast enough platform to compete with native applications, thats no longer a question I think about.

People think the web is slow because opening facebook.com takes >200 requests loading >4MB over the network of which a large chunk blocks any rendering (and all of which interferes with it). Build a web application which loads its UI from disk, caches application data, only touches the network for updates, does not block rendering waiting on bundles of JavaScript and uses proper practices for JS / dom manipulation (use transforms over animating top/left, dont attach process heavy scroll handlers) and your web app will be as fast as a native app