Set up a mission with plenty of AI on both sides, then remove all grouping from them so they are all individual squads. Give each a waypoint with only open terrain between them and the WP. Performance should be excellent regardless of the number of AI. Be sure to keep both sides at least 200m away from each other to prevent them seeing each other and altering the test.
Do the same with them in a city or other complex terrain. Performance should be as bad as when in combat.