Yes, that's what I meant by "strictly slower". At best, it can be optimized to something similar to what RAII can give you. RAII never has the overhead of the bad case.
I don't want to be excessively picky, but you said that is has "unavoidable runtime overhead", and that's true only in its rarest form (defer within a loop), which is a feature which you can't implement in RAII. IOW, defer is a superset of RAII.
In all other cases (which is almost all usages), it is semantically equivalent to RAII, so the language doesn't force any runtime overhead. The only difference is that the compiler is less mature than an average C++ compiler, but this is an implementation problem, not a design problem.
RAII has no overhead because it is a pattern designed within the context of a zero-overhead language. defer allows you to implement a superset of cases that RAII handles, including those with runtime overhead.
defer isn't a superset of RAII. defer is function-scoped. RAII is block-scoped. If you want to run code at the end of a block, you can do that with RAII and you can't do that with defer—and note that this is what causes all the codegen issues with defer.
