I came across a blog by a company called Illation. What those guys do is compare performance of several rule engines available on the market: Drools, ILog, OPSJ and Jess.

The stress tests cover different aspects, for example:

• Rules firing time
• Data load time
• Memory usage
• Pre-run memory used
• Post-run memory used

The test results available on their blog for the wide public. The team also makes business rules, object model and datasets used in their stress tests available for download if someone wishes to repeat the tests. Some of the results look very interesting