This allows you to run my library without having to trust me or my code.
Isn't that what Docker, firejail, nsjail, flatpak and others already achieve...?
No
Yes
A standard Rust program has full access to any filesystem, network, etc resources that the user running the program has. WASM sandboxes these resources by default, only giving the program access to exactly what you explicitly allow at runtime.
Performance is going to be fast enough in most cases, you get the extra sandboxing, and it runs multiplatform. I'm not speaking from experience but just my understanding, so I may be incorrect.