We upgraded from php4 to php5 last week. Zero errors with UBB. But as for speed. We have not totally identified the issue yet but our load times are 20% longer now with php5.

To be fair though ubbthreads.php is loading at about the same speed. Our template has taken a big load hit though.

We use http://www.octagate.com/service/SiteTimer/?Target=AJAX to test the load speeds.