I am experiencing performance problems using drools with a very simple use case. I have 50-100 rules, 5-10 facts, and assert facts and fire the rules hundreds to thousands of times per second. I have attached a basic example of the problem I am encountering. You'll see that as it loops, creating new WorkingMemory objects, asserting facts, and firing rules, the time to do this takes longer and longer until it gets to an unusable state. After running this through JProfiler, I found the hot spot to be PrimitiveLongMap. When the RuleBase is cached and new WorkingMemoryImpls are created (as the preferred approach seems to be), the generated FactHandle ids continue to increase. The problem is that as the number gets large, say over 1 million, the PrimitiveLongMap seems to spend all of its time creating Pages, or alternatively, iterating through all of the pages to return the collection of values when WorkingMemoryImpl.getObjects is called. I was able to fix this for my use case by simply replacing PrimitiveLongMap with HashMap. Of course, I'm assuming PrimitiveLongMap was used to replace a HashMap for a reason (very large number of facts? Long object creation?), so this is probably not a long term solution for the drools project. So I see three other options: 1. Fix PrimitiveLongMap. I haven't spent the time to try and understand what type of work this would require. 2. Replace PrimitiveLongMap with a different implementation (perhaps one of the ones listed on http://blogs.codehaus.org/people/mproctor/archives/000890_primitive_map_implementation_and_benchmark_comparisons.html ). 3. Generate fact handle ids on a per working memory basis (as shown in the WorkingMemoryImpl.java included). I don't know if the RuleBase ever uses the FactHandle id in a global way or always just passes the FactHandle object through to a WorkingMemoryImpl, so that could lead to potential problems. Let me know if any additional details are required.