It's not the hashing that's slow. It's the lookup that is.
By "different hashing scheme" I meant different strategies in resolving hash collisions, likes double hashing, internal hashing, cuckoo hashing, and so on and on. Maybe one of such alternative strategies is more fit for the needs of dmd compilation. (I think that currently the Python dicts are using a hashing strategy different from the built-in dictionaries of D. The Python style of hashing was implemented in D some months ago, but I don't remember what happened to that project later).