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Why would you think that? Rust is certainly a little bit slower than C/C++
Everything I've read says that you're right in theory, but in practice, rust rewrites tend to outperform. At Mozilla's level, though, I wouldn't necessarily expect that to hold water. They should be able to write great code in either language.
I vaguely remember reading here about a company that slapped a backend together in Python. When they took off, the compute costs stung, so they rewrote in Go. Saved most of the bill. Some time later that started creaking, so they rewrote again. In Python, but managed to save most of the bill again.