bp_forum_pagination_count() uses the $forum_template global, which is why it outputs instanteously.
bp_get_forum_topic_count() calls bp_forums_total_topic_count(), which has a do_action( ‘bbpress_init’ ) line. Try commenting out this line in /bp-forums.php and see if the memory problems persist.
If it the memory problems persist, then it’s a problem with the BB_Query instance, which I have no idea how to debug!
