The CSSU-Thumb2 repo (see http://talk.maemo.org/showthread.php?t=84829) contains Fennec 17. That is the fastest version I've tried so far (and I tried a lot of them).
I did switch off Javascript, which seems to make a big difference so maybe that is the main reason.

The difference may also in part be due to the Thumb2 improvements, so I am not sure if simply compiling Fennec 17 normally is useful.