The fastest way to merge large dictionaries is to introduce an intermediate object that behaves as though the dicts are merged without actually merging them (see @Raymond Hettinger's answer):

from collections import ChainMap

class MergedMap(ChainMap):
    def __getitem__(self, key):
        result = []
        found = False
        for mapping in self.maps:
            try:
                result.extend(mapping[key])
                found = True
            except KeyError:
                pass
        return result if found else self.__missing__(key)

merged = MergedMap(myDict1, myDict2)

Whether it is applicable depends on how you want to use the combined dict later.

It uses collections.ChainMap from Python 3.3+ for convenience to provide the full MutableMapping interface; you could implement only parts that you use on older Python versions.