Rust is not inherently slower but then again neither are C and C++, but in practice all of those tend to be less efficient than realistic Java systems. Rust is displacing C in contexts where Rust's less than amazing performance are not noticeable in contrast to C's also-not-amazing level of performance. And I also think there is a cognitive bias under which a developer will reach for Rust to supplant a legacy C program, because that developer reflexively dislikes managed language runtimes.