The prefetch will always take same amount of data no matter what the size of python object in question is. That leads two results. 1) You can miss if your cacheline size is less than python object size. 2) You miss if your python object is not alligned at the cacheline and comparison is with the part that is in the next cacheline side.

If these two condition apply then there is limit of 2 on speed up and increasing python object size reduces the effect of prefetch which still helps.

Another about prefetch distance I'd pick 5 based on his testing data. 1) It has no downside on current tests. 2) Due to prefetch constant size any prefetch that hurts because you pick distance of 5 instead of 3 is unlikely to have any meaningfull effect at any prefetch distance. 3) It allows some improvement in comparison speed. Like if there where JIT on python side to specialize the test. 4) It allows a nice factor of improvement on CORE execution speed relative to memory latency. 5) It allows you to handle situations of page misses better.