it will require more processing time but not much. The box has a rule order. So it does check the rule was the most requirements, does all the checking and then fallbacks to rules with less requirements.
So, if you have currently a rule with just a url matching and add anoter rule with url + header-map it will require more processing as before the box did not have to look at the header-map.
If you do the reverse and already have url + header-map and add a rule with just a url, the box will continue to check header + url and if the header does not match but the url does, it requires a small extra step to fallback from rule 1 to rule 2.
Gilles.