This is somewhat true. It's the maps that are the problem not the action. The maps which suffer from performance drops run poor regardless of whether there are enemies on screen or not.
So it goes along with the pc thread where looking at fires and being outdoors will cause 20 or more fps drops while dozens of enemies leave the frame rate relatively unaffected. Is this an idtech problem or are some of the maps just poorly designed? This does suggest that they can't fix the problem through patches unless they actually change some of the renders or remove the offending effects themselves.