Earlier quoted context omitted.
specifically, it sounds to me like ATI/AMD's rather-notoriously-poor OpenGL performance in action... another way to rephrase OP to take the emphasis off hardware generation (which makes no sense) would be "when I switched from NVIDIA to ATI I lost 40% of my performance"... which is still an experience many people have today with OpenGL on the AMD cards.
I could be wrong (I never really got into gaming as GMA950+Linux was an exceptionally bad combo for it) but I thought source was exclusively DirectX back then.
Edit: I'm thinking of the GoldSrc engine used in Half-Life