3.2.2.1.3.1 Notes about the Implementation of Compiler Macros

Although it is technically permissible, as described above, for eval to treat compiler macros in the same situations as compiler might, this is not necessarily a good idea in interpreted implementations.

Compiler macros exist for the purpose of trading compile-time speed for run-time speed. Programmers who write compiler macros tend to assume that the compiler macros can take more time than normal functions and macros in order to produce code which is especially optimal for use at run time. Since eval in an interpreted implementation might perform semantic analysis of the same form multiple times, it might be inefficient in general for the implementation to choose to call compiler macros on every such evaluation.

Nevertheless, the decision about what to do in these situations is left to each implementation.