> I am not sure whether N900 keeps microB loaded on the background all the time or what’s causing the difference.

microb is indeed preloaded. in fact, both the ui and the browser are preloaded. it’s possible with a bit of effort to disable this, but you shouldn’t find it helping fennec’s startup time significantly (it should of course hurt microb’s start time to give you slightly more useful comparison numbers).

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> microB must do the rendering somehow different… some other trick?.

The trick is that microB is two processes, the user interface (responsive) and gecko (blocking).

> However, until Fennec reaches the smoothiness of microB, there is no way I would switch to Fennec over microB.

Roughly this is coming, it’s under the IPC / multiprocess work. The first fruits of that are out of process plugins which have kind of arrived (at least on mozilla-central).