Rust libraries are designed to be fast when optimized with LLVM. Rust has a lot of layers of abstractions, and they're zero cost only when everything is fully optimized. If you look at unoptimized miri execution, or insert code-level instrumentation that gets in the way of the optimizer, these aren't zero cost any more, and overheads add up where they normally don't exist.

> Important note: Miri is not intended to accurately replicate optimized Rust runtime code. Optimizing for Miri can sometimes make your real code slower, and vice versa. It’s a helpful tool to guide your optimization, but you should always benchmark your changes with release builds, not with Miri.