
-one addditional UPDATE is showthread.php
-one additional SELECT (of 2 fields only) for each thread or subforum on forumdisplay.php and search.php pages
-one addditional UPDATE in newthread.php
-two addditional UPDATE in newreply.php
-"mark forum read" works using one UPDATE
-and "mark all forums read" using one UPDATE for each forum

So, in theory forumdisplay.php will work little slower, but i can't notice it using eyes only