The iSeries Java Toolbox JDBC driver would be your best option. Performance problems are usually caused by an underpowered server, an untuned database, or poor application design and not the middleware. I highly recommend you attend the iSeries SQL Performance Workshop. If you go to that page and following the links for Online DB2 Education, you'll find some of the Performance Materials from that Workshop (no lab exercises however).
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This was first published in April 2003