Slow performance with 9.3-110x JDBC 4

I have a java program that does bulk insertions (with transactions and PreparedStatements) in a postgresql database. I have a performance divided between two and four when using 9.3-1100 JDBC4 and 9.3-1101 in comparison with postgresql-8.4-703 JDBC4 independently of postgresql version.

 Time(ms)                PG     8.4.7            9.3.4
Driver
8.4-703                          193390         188379
9.3.1101                         349404         344344

Does anybody else saw such a difference ? What reasons can explain it ?

Another point, is that the bulk insertions are prerformed by parallel processes and 9.3-110x JDBC4 drivers appear to scale poorly.

Thanks for any help.