Can anyone clarify what exactly microb is?

It's clearly firefox derived, and has been optimized for maemo. Indeed it works very very well

I was surprised to see a rather large performance difference between fennec and microb. Since their both using the same rendering engine (aren't they?) this implies it's probably just a matter of time before fennec catches up

Or is there something else at work here -- is microb using any proprietary components perhaps with additional video acceleration?

Only reason I'm wondering is that microb will probably remain fairly similar, whilst fennec will evolve rapidly, and in the world of the web that could become important -- for example youtube's recent foray into html5+h.264 -- things that microb doesn't (yet?) support (I think)