I guess I was thinking more of multithreading as an umbrella term for parallelism but I see your point. If you squint, SIMD and multithreading are the same problem. What I mean is, if you have a computable function you want to evaluate on a finite amount of bits, the problem of how to physically layout and schedule the computation to minimize latency is very similar to maximizing throughput.

From a circuit complexity standpoint, you could evaluate a wide but shallow circuit to evaluate the function in a nanosecond, or a deep but narrow circuit that takes eons. Whether parts of that circuit are evaluated synchronously or asynchronously is immaterial, although synchronous computation does seem easier to reason about and the UX is more user-friendly from a programmability standpoint.

I agree with you the fundamental limitation is the speed of light if you are width-bounded (i.e., if physical space is the dominating constraint).