^ You need to play Bloodlines. Source really showed it can't handle high amounts of data in a single map.
With the sheer amount of npc's, npc speech, quests, side-quests, items required by most rpg's, it fell flat on its face with a 2gb memory footprint. lol hardly flexible... CryEngine and Serious Engine 2 are flexible.
Even HL2 had to rebuffer every 5-10min. Source can't be overburdened at all, you really need to finetune everything even without using any of the advanced effects.
But yea, CSS itself doesn't really lag. 50-60fps easily at 1280x960 with dips only during intense firefights or smoke. But CSS is essentially just a set of empty maps without much content.