That's JMS for you, it mostly comes down to I/O bottlenecks.
You might get better results from MySQL, especially if it is located on the same machine. Oracle is traditionally hard to tune (IMHO) unless you have a Oracle expert.
thanks genman, I'm currently in the process of switching to OCI instead of the thin driver; some fairly derogatory comments towards the lineage of the thin driver suggested the switch might help.
After that I'll try MySQL - the price is certainly better