Keep in mind the Jaegermonkey project, which will greatly improve javascript performance is still under heavy work. So as of today, the javascript performance is still significantly behind chrome. (http://arewefastyet.com/)
That being said, javascript performance currently has little impact on user-perceived speed, and the "Reducing I/O from the main thread", GPU accelerated rendering, and startup/shutdown perf improvements are already causing incredibly noticeable perf gains.
That being said, javascript performance currently has little impact on user-perceived speed, and the "Reducing I/O from the main thread", GPU accelerated rendering, and startup/shutdown perf improvements are already causing incredibly noticeable perf gains.