I assume the M1's SIMD registers are wider/more numerous than just the couple of size_t registers used for the loading/masking/accumulating inner loop in your run_swtches().

You can speedup the code by unrolling your inner loop a few times (try 4x or 8x) - it does mean that your overflow prevention limit is lowered (to a multiple of the unrolled grouping number) and run a few more times. But the speedup offsets the increased bookkeeping.

A version I played with showed increased speed by saving the in-progress accumulation in an array and then doing the final accumulation after the main loop is done. But that may be due to the CPU arch/compiler I'm using.