c++ - is returning a const std::string really slower than non-const?

In another question a user made a comment that returning a const std::string loses move construction efficiency and is slower.

Is it really true that assigning a string of return of this method:

const std::string toJson(const std::string &someText);

const std::string jsonString = toJson(someText);

... is really slower than the non-const version:

std::string toJson(const std::string &str);

std::string jsonString = toJson(someText);

And what is the meaning of move-construction efficiency in this context?

I've never heard of that limitation before and do not remember having seen that in the profiler. But I'm curious.

Edit: There is a suggested question asking: What is move semantics?. While some of the explanations of course relate to efficiency, it explains what move semantics means, but does not address why returning a const value can have negative side effects regarding performance.