One hopes that the optimizer has arranged to check the bounds at the start of the loop, and not in it. Apparently Rust's doesn't, yet, but it seems usually to happen to overlap the checking it does with other stalls, so it might not usually matter much.

The actual bounds checking instructions are often negligible in terms of overhead, but it can inflict damage by inhibiting other optimizations and loop optimizations, like reduction or vectorization.

Low-hanging fruit. But there are still bigger fish to fry.

Lower-hanging bigger fish?

Rust isn't mature yet. Give it ten years. It will either exceed C performance, or fade away. It's still too early to say which.

What's the bigger things that need solving in Rust?

Compile speed. Feature completeness. Tooling maturity.

Compile speed is the one I agree on, the others I haven't really run into.