The language itself isn't garbage collected. It only allows for the option of language implementers to implement their own custom garbage collection scheme as a secondary option.
All C++ compilers must continue to support manual memory management as the primary mode of operation. End users may choose to use garbage collection if they want to and the compiler and libraries support it, but they are not forced to.
Therefore, C++ is not now considered slow. It's actually faster than before thanks to things like rvalue references and move semantics.