Having a dense array is not, as it seems both you and chippiewill imagine, somehow a unique property of ordered dictionaries. An unordered dictionary is free to use exactly the same implementation detail.
The choice to preserve order is in addition to using dense arrays. The OrderedDict must use tombstones in order to preserve ordering, and then periodically rewrite the entire dense array to remove tombstones, while a hypothetical UnorderedDict needn't worry because it isn't trying to preserve ordering so it will be faster here despite also having the dense arrays.
"iterating an array of 100 items that fits in one cache-line will be faster"
On today's hardware a cache line is 64 bytes, so fitting 100 "items" (each 3x 64-bit values, so typically total 2400 bytes with today's Python implementation) in a cache line would not be possible. A rather less impressive "almost three" items fit in a cache line.
But to be sure the dense array is faster for this operation, the problem is that that's not an optimisation as a result of being ordered. It's just an implementation choice and the UnorderedDict is free to make the same choice.
