I don't think the 6 weeks vs. 3 years tells the full story. C++ collects up a whole truck load of changes and waits up to 3 years then dumps all of them in one go. Whereas Rust releases them individually, waiting up to six weeks after they've done baking in the nightly compiler (though they've likely been in development for years before that).

That's not to say Rust is necessarily slower paced than C++ either, just that a raw number on release dates aren't helpful by themselves.