It's within 5-10% on average, sometimes faster, usually a little slower, very rarely slower by much. The person you're responding to said "approximately". I feel like this is needless pedantry.
Rust also allows you to make architectural decisions in the name of performance that would be completely unmaintainable in C. See: the Servo project.
Further, it should be possible for Rust to eventually reach a state where it's faster than C because of the lack of pointer aliasing. This opens up a whole host of optimization opportunities, allowing it to get closer in performance to Fortran (which is intrinsically the fastest of the bunch).
What makes you say that? Optimized machine code is for the most part optimized machine code, and unsurprisingly benchmarks tend to come down about even. The bunchmarks game currently has them down as
C vs. Rust: 6 wins for C, one draw, 3 wins for Rust
C++ vs. Rust: 5 wins for C++, two draws, 3 wins for Rust
The wins one way or another are also not by particularly large margins.
This may go on to shift marginally in Rust's favor once a soundness bug related to non-aliasing of references is fixed in LLVM and the compiler can safely leverage some guarantees that Rust provides that C and C++ cannot.
