If you are using a Raspberry PI and Java: Go install the latest Java 7 SDK from Oracle for the PI.
Raspberry PI announcement
This version has been compiled for the PI with hardware float set and some other optimizations. I measured a huge decrease in processing time (transactions that were taking 4-5 second are now subsecond) with the new version.
As always, Your Mileage Will Vary, but it's well worth taking the 10 minutes out to download and set up the new version.