Memory traffic is certainly the biggest problem that the array paradigm presents for implementation, yes. I'd quibble with calling that "poor locality": when working with large arrays, if any part of a cache line is accessed it's very likely the entire line will be used in short order, which is the definition of good locality as I understand it. The issue is simply high memory use and a large number of accesses.

I think it's an oversimplification to say SPMD is a better model full stop. Array programming has the advantage that the implementer optimizes a specific function, and its interaction with the rest of the program is much simpler: all the data is available at the start, and the result can be produced in any order. So things like multi-pass algorithms and temporary lookup tables are possible, particularly valuable if the program isn't spending that much time on arithmetic but rather "heavy" searching and sorting operations. Not that I claim NumPy or CuPy do a good job of this. Sections "Fusion versus fission" and "Dynamic versus static" are relevant here, I think: https://mlochbaum.github.io/BQN/implementation/versusc.html#...