Nonlin ar partial differential equations through a large continuum are expensive. Even if you can scale this particular example, it doesn't refute the point, the universe does a tremendous amount of compute that we don't know how to exploit.
I think it is totally possible for the computing to take non-zero time, but we observe it in zero-time as our consciousness only steps forward only with each iteration of computing the world state. So we observe zero time reality computations.