You picked a particularly poor example. The application compatibility layer does not incredibly hamper the OS whatsoever. First of all the ACL is only active when known executables that have problems with the newer versions are running. As an educated guess it would not take more than a millisecond to ID the .exe. Secondly what the ACL does is fairly benign - what it amounts to in simple terms is that it simply uses a different code path for a very narrow set of functions. The cost associated with loading an 'extra library' is so marginal that you would find it hard to measure.
