While measuring for bottlenecks in my application I noticed that a considerable amount of time was spent on executing a really simple query. Then after focusing a little bit more on the queries I realized that they are not scaling well.
Btw the Drools version that I'm using is 7.0.0-SNAPSHOT but by doing the same tests on the newest version (which at this point is 7.22.0.Final) the same problem is still present.

All the details can be found in this StackOverflow question.