The default should be to show the password input for normal users. Chances are, most corporate users are 1) using a fast connection paid for by their employer and/or 2) used to shitty stuttering UI, so it will be less of a problem if it takes a couple of seconds to load their 3rd party auth provider.

If the number of auth providers are small and tied to specific domains in the email address, perhaps you could speed up the process by pre-loading a hash table of domains => auth providers.