Currently, most mixin engine APIs that accept target types also allow the caller to pass a generated (concrete) mixed type. Then, within the API, the engine detects that the type is generated and treats it differently.
For example, ObjectFactory.Create (Type) will, when passed a concrete type, instantiate that type rather than generating a new one. MixinConfiguration.GetClassContext (Type) will extract the context that was used to generate the concrete type, etc.
While this is convenient, it costs time, and is probably not often used. By removing that feature (and always assuming a target type), some performance could be gained.