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Software is slow because of bad design, not slow languages
Yes and no. It's not usually the fault of the programming language.
But programming languages have a unique ability: The design of the language itself can and will instill bad or good habits into the code of oneself. These languages are designed pretty deliberately, so those habits are a direct consequence of the design of a language. In fact, Sean Goedecke's own C-to-Rust example makes this point implicitly: Cantrill's port got faster not because the redesign was really clever, but because Rust's borrow checker and standard library actively steered him toward a more cache friendly Binary tree. There are probably many more examples out there that had a real world impact.
There are no bad languages, every language has its place somewhere. But design decisions matter.