The article is mostly about exemplifying the various leve of optimisation you can get by moving “hot code paths” to native code (irrespective whether you write that code in rust/c++/c.
Worth noting that if you’re optimising for memory usage, rust (or some other native code) might not help you very much until you throw away your whole codebase, which might not be always feasible.
It should be about the same, though the main differences are likely to be caused by the speed of the QR code generator, and the PNG compressor.
But assuming that the hypothetical C and C++ versions would be using generators and compressors of similar quality, it performance characteristics should be similar.
The big plus(es) to using Rust over C/C++ are a) the C and C++ versions would not be memory-safe, and b) it looks like Rust's WASM tooling (if that's the approach you were to use) is excellent.
(As someone who has written C code for more than 20 years, and used to write older-standard C++ code, I would never ever write an internet-facing server in either of those languages. But I would feel just as confident about the security properties of my Rust code as I would for my Java code.)
