Combo Edit Processing times

Question for the group:

We have multiple BUs, and each BU has it's own Combo Edit Rules.  Some Rules are the same, but some are different, therefore we will have over 100 combo rules once all BUs come online.  When the edit process runs, does it check only the Rules associated with the respective BU, or does the process have to evaluate every rule for every BU?  In other words, if all 25 BUs have the same basic Rule, does combo edit take the time to evaluate the rule 25 times, or does it just select the BU-specific rule?

Thanks in advance for any input you have,