Cython just transpiles to C. It's not "as fast as C", it's as fast as the runtime support structures it uses, and the communication with Python allows.
C/C++ compilers might be able to generate more optimized native code than what rutsc does though. Actually, this is a question: how good is rustc with numerical / math intensive code? For instance, does it implements loop unrolling and SIMD vectorization?
For example, in my case I always need high-performance code to work with strings loaded from loads of CSV files. That includes: merging strings, matching them, comparing them. Loop unrolling/SIMD would not really help here, while an ability to write safe, checked code fast - would.
On the other hand I do need the pythonic dynamics, so that's what I stick to.