I've covered this before in articles such as this: https://neuml.hashnode.dev/building-an-efficient-sparse-keyw...

You can make anything performant if you know the right buttons to push. While Rust makes it easy in some ways, Rust is also a difficult language to develop with for many developers. There is a tradeoff.

I'd also say LangChain's primary goal isn't performance it's convenience and functionality coverage.