There have been many improvements in the SQL connector regarding security and stability after release 5.4. Several parameters (Connection string properties) that makes it possible to customize the connector have also been added. It is likely that this have a small cost in performance. However our own experience in the Qlik R&D SAP connectivity team is that the SQL connector still has a very good performance!
Maybe it is possible for you to use the Connection string properties to customize the connector and get a better performance. You can read more about it in the online manual.