Modern CPUs are extremely capable superscalar processors. In order to properly feed them, you must design with SIMD in mind. Even then, you can almost never rely on compiler auto-vectorization, and you must use intrinsics yourself. This is necessary even for such simple things as automatically opening doors when you move near them. In other words, many many things that are never even done on the GPU.

Of course, after release, the biggest improvements come from outside the game dev team. The point is not to forget all the improvements very carefully and skillfully baked in before release, just because consumers don't see the delta.