Also things like having clear ownership rules, using containment instead of pointers, not copying data around that doesn't need to be copied, operating on slices & views of large collections instead of creating a whole new collection, etc.

C++ devs who work in high-performance industries (like games or finance) know how to write fast code. C++ is not used in most consumer software these days, and even if it is used, you often see slow practices like chasing pointers everywhere, using inheritance instead of generics, etc. Even in big-data, an industry that you'd think would really benefit from fast code, the primary open-source tools are in Java. You could probably recover a 2-4x speedup - two generations of Moore's Law - by rewriting Hadoop in C++, but nobody (besides Google, who is happily crunching data with MapReduce etc.) has bothered.

That's because writing fast, correct C++ code is hard, and it tends to be more economical to write in Python or Node.js and then rent more VMs. If it is no longer economical to rent more VMs, you can certainly recover a lot of that performance, but until we get languages that let you use the performance tricks of C++ but have the safety & ease of use of Python (I have high hopes for you, Rust), it probably won't happen.