Arrays are generally faster than Vectors, because they are not mutable, and because they don't need to allocate more memory. Is the same true for Variables?
Would:
let mut x: i32 = 1;
be slower than:
let x: i32 = 1;
Does the compiler remove the unnecessary muts in the code?
You can test such things on rust.godbolt.org, but remember to always add -O to the flags field on there! Otherwise godbolt gives useless noisy result.
In Rust variables don't even exist when the program runs, so how they're declared means very little. The optimizer will figure out how the data is used, and optimize for specific usage.
Also in Rust all exclusively owned values are always mutable. mut is just a weak lint for humans, but it's meaningless and irrelevant for the compiled code. Note that this is different from &mut which is strongly enforced and does affect optimizations in some cases.