Comment by gmueckl

This is valid for sequential scanning of the data. The CPU will fill whole cache lines at once with the arrays that do get used and the algorithm touches all the field instances in the array.

Now think about random access to single struct instances instead: the CPU loads a cache line worth of data for each field and uses only one element out of the whole cache line. This is much worse than a compact structure representation of the same data.

SoA is not universally better.