JavaScript is getting faster and so is cheap hardware.

There are three levels of execution with Firefox - rarely used functions are interpreted, functions used fairly frequently get a quick compile and hot code gets a further level of compilation after which it runs extremely fast.

Basic coding errors can dramatically slow your app down, for example changing type information gets your lovely fast compiled code kicked back down to the much slower interpreter, but I don't think we're far from the point well coded JavaScript on cheap hardware will give a perfectly acceptable experience.

Also remember the HTML5 canvas uses hardware acceleration where possible. Most of the grunt work of a game is in the rendering not the game logic, I don't think games are going to be the big problem for FFOS many imagine they'll be.

If I were going to make an app for FFOS I'd consider using the canvas and a library like KineticJS rather than the DOM so I could make the UI transitions and animations very slick.