Maybe it's easier to see if we're explicit about what the rules are: OrderedDict (now the Python dict) is exactly the same features as a hypothetical UnorderedDict except OrderedDict has the additional constraint that if we iterate over it we get the key/values in the order in which they were inserted, while UnorderedDict can do as it pleases here.
This means OrderedDict is a valid implementation of UnorderedDict. So, necessarily OrderedDict does not have, as you claim, "improved performance over an unordered dictionary". At the very worst it's break even and performance is identical. This is why it's remarkable that Python's previous dict was worse.
But, that's a pretty degenerate case, we can also see that after deletion OrderedDict must use some resources ensuring the ordering constraint is kept. An UnorderedDict needn't do that, and we can definitely do better than OrdererDict.