Concurrent birt reports generation slow

I'm having a problem with concurrent birt reports generation in a JBoss environment. Sometimes, when more than a user generates reports at the same time (even when the report designs are different), reports take a very long time to complete. Reports that usually completes in less than 5 seconds lasts 2 minutes or more! Taking a dump of the threads currently running, I've noticed something strange:

org.eclipse.birt.report.engine.data.dte.ReportQueryBuilder.build(ReportQueryBuilder.java:256)
- locked <0x45a228> (a org.eclipse.birt.report.engine.ir.Report)
org.eclipse.birt.report.engine.data.dte.ReportQueryBuilder.build(ReportQueryBuilder.java:237)
- locked <0x45a228> (a org.eclipse.birt.report.engine.ir.Report)

could this be the cause of it? If yes, how can I deal with it?

thanks a lot!