update takes to long

I have a sql statement that really could use som tuning, but I do not know how. It is updated by a self join, and contains appr 20000 rows. The statement executes on almost 3 minutes. I have rewritten it to an anonymous block, but I only reduce the time by half, which is not enough. Any help would be appreciated. Thanks

UPDATE a

   SET (sortorder) = (SELECT COUNT (sortorder)

                      FROM a b
                     WHERE b.PATH <= a.PATH)