rlm_cache and ntlm_auth

Probably a simple question, but I've Googled it and I can't find the answer. Is it possible to wrap an ntlm_auth backend in rlm_cache? Our active directory is frequently quite slow. If we were able to grab at least some of these authentications from the cache I think it would help a lot. I don't know exactly what data is returned from NTLM, is it cacheable or does it have to be fresh each time? By the fact that I haven't found any mention of anyone doing this, I suspect it's probably not possible. Thanks, Jonathan