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"Rules itself is written with performance in mind, thus the system retrieves the rules from cache and evaluates them without the need for any additional db queries or such. So the performance impact of Rules mostly depends on which rules one configures.
While some developers and site builders see the processing that's done by Rules and believe that it will slow down their system too much, side-by-side comparisons are still yet to be posted."
A related discussion: Anyone done Rules performance checks?.