If you watched the E3 movie, you could see that HL2 was choking in some scenes, and it was running on a 9800Pro P4 3.2GHz
I bet this has more to do with the sudden delay than anything else. Valve probably considers the game "done" (hence their recent comments that the delay was untrue), but Vivendi is really concerned that it won't run well on the majority of systems out there. Perhaps they want better performance scaling added. Maybe they counted on code optimizations that didn't end up helping much. The Nvidia flap is another problem, especially considering all the Nvidia card owners out there. Maybe Nvidia is having trouble with their Det50 Optimizations. Or perhaps Vivendi figures that people will upgrade their systems during the holiday season, which is the best time to release a resource-hungry game like this.