With using static variables in some cases it is possible to improve performance
The changes we have done are here: http://git.luns.net.uk/moodle.git/?p=moodle.git;a=shortlog;h=refs/heads/cvshead-optimisation

Each commit message contains profiling data indicating how the change affected performance. On average, suggested changes reduce the page generation time by 200ms (tested with moodle2/mod/forum/view.php?id=x) and increase memory usage by 1%