Preventing work for compiler developers is the worst possible argument for extending a language. Once a language is extended, this can never be rolled back and every person that afterwards wants to use the language may have to learn about this extension at some point. It makes the language harder to learn, harder to use, harder to read, harder to maintain. In programming languages, less is more. Delphi-style Pascal is already way more complex than it should be as it is.
Furthermore, adding the extension in a way that the compiler doesn't even check whether it's correct is even worse. At least (modern) C++-compilers, and presumably D-compilers too, verify that const methods indeed do not mutate the state of the instance. The most important parts about a programming language is that it makes it easy to understand an already written program, and making it difficult to write buggy code. Performance is generally a very, very distant third (or fourth or fifth or sixth, coming after "making it easy to write programs", "making it easy to automatically analyse code", "making it easy to quickly parse/compile". ...). Having all of the aforementioned properties generally also automatically makes it easier to both write and generate (correct) fast code.
Finally, whole-program optimisation information could also be generated for dlls and objects and shipped together with them if that would be critical to performance when using them.