I just profiled one of our test assemblies (Unit tests only, nothing hitting the db). Mocking accounts for 196 seconds out of 357 seconds. The culprit is System.Reflection.Emit.TypeBuilder.CreateType(). This doesn't come as that much of a surprise to me, but still... is there a faster way to do this? Oh and yeah, the results of CreateType are cached so its only the first time you create a mock of a type that you take this hit, but still... that can really slow down TDD.
And no, I'm not saying we should stop mocking... just raising awareness.