it depends. Oracle´s FIRST_ROWS hint leads into nested loop joins (on indexes) which could be a big decrease of performance. If you have Oracle 9i or higher you could try FIRST_ROWS(100) which will process faster while using hash joins.
I would optimize the query in Oracle first (sqlplus or toad, explain plan etc.) w/o hints. Keep in mind that you will retrieve all rows in the QV load, not only "first rows". So, processing time of the whole result set is crucial.
Maybe something is wrong with your join condition (between tables A and B) and/or missing indexes...