Sure given the laws of physics there is a finite amount of computation that can be done in a fixed volume. We are still a long way from reaching such limits and the human brain is very far from such limits.

One of the reasons we should fear a super AI is that it will be constrained by the laws of physics and so will want to use all the resources within its light sphere efficiently. Humans are not an efficient structure for computation given the how we evolved so any unconstrained AI would likely not hesitate to reorganise us into a more efficient structure.