I know from experience that switching tree model in Saxon 6
changes performance pattern. Instead of (or additionally to)
having two tree structures, it would make sense to use
different tree structures for node-sets and RTFs.
I think these optimizations can be achieved without imposing
restrictions on the user. Saxon 7.8 already does better than 6 in this
area (though this is masked by slower performance elsewhere). I think a
lot of the reason that xt is so fast is because it's very slick at
building trees. Many more improvements are possible given the time and
effort.
Incidentally, final result trees and RTFs are quite different. Final
result trees never actually exist in memory, nodes are serialized as
they are constructed.
Michael Kay
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list