> Any optimization you could do in Rust is probably easier in C++
That's kind of funny in light of the history that certain optimizations in web layout engines were attempted, unsuccessfully, in C++ multiple times and ultimately they invented Rust to make them easier.
The facts on the ground probably have more to do with improvements to the C++ code being obliged work as deltas against existing C++ code, where the Rust code was a complete re-implementation, thus not constrained.
Both C++ and Rust are today different languages from when that project ran.
Another subtle consideration is that with long-lived, highly-backward-compatible languages like C++, you'll have a bunch of people on your project who still write C++ like it was ten or twenty years ago (because that's when they learned it), and so bring down the code quality of your project. Whereas choosing a new language (like Rust at the time) means that everyone who claims to be able to write it at all, is working from the same, recent set of language idioms.
That's kind of funny in light of the history that certain optimizations in web layout engines were attempted, unsuccessfully, in C++ multiple times and ultimately they invented Rust to make them easier.