Each length is calculated twice, so I can't even tell without testing if caching would help or not. And the complexity is just O(n * log n), the group lengths are not independent of the list length, they are made of the same items.

It has approximate time complexity O(n log n). ( I already thought so. But I checked. ) Your C or assembly solution might have time complexity O(n), but will very, very probably also have O(n log n).

No, length is not cached. But even without, time complexity is something like (O m n log n) with m the length of the longest group. That's approximately a small constant except in extreme cases. Caching length would be a good thing here.

I just checked, this looks covered.

I expect quickcheck to test for the empty list !

Tests do not check myMinimum on empty Foldable (I assume it should be Nothing)

Is that better now ? (note : I don't think tests are updated when a kata is retried, you'll have to check the kata details ..)