Reasons for speedup?

Sprinkling 'restrict' through in your C code is not even an option; it would be a disaster. Sooner or later you're going to accidentally violate the restrict constraint, the code will start misbehaving in weird ways (but only with certain optimization levels and compilers), and since no sanitizers address this feature, you'll have a nightmarish time trying to figure out the bug.

For this reason and others, the performance ceiling for single-threaded Rust code is potentially quite a bit higher than for similar C or C++ code. A lot more compiler work is needed though.