I do not have hard numbers - however keep in mind that practical "performance" also includes memory bandwidth and total RAM, this is especially a consideration for the ColdString type - a billion ColdStrings is 8GB of RAM, but a billion MSVC std::string needs 32GB of RAM. Rust's std::string::String is of course much faster than any of the std::string implementations because it never has the SSO case to consider - but for non-empty strings it's also more memory bandwdith and RAM needed.