That is why I have doubts about their scalability. There are fundamental physics reasons we were able to scale digital computers and were unable to do the same to analog ones.

That's certainly true. But - the fastest possible machine to compute the sound of a guitar string reverberating in a concert hall is a guitar in a concert hall. The world itself does a tremendous amount of compute - the question is whether it is useful compute. I don't think this has been explored nearly enough.

Why is it the fastest possible?

Wouldn’t you be able to run a higher clock-speed simulation, and therefore compute the sound ahead-of-time, on a digital device?

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.

This assumes that the universe is computing.

But there is some hypothesis like MUH that sees reality as a sort of "recording" that we just experience.

>Why is it the fastest possible?

Why? because the "universe" takes zero time to compute it. In fact all of reality is computed in zero time. How can you beat it?

TO continue the above comment..

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.