Nick Babcock writes about Rust, WebAssembly, performance tuning and API design, based on his own projects.
Commenters cite it for WASM and SIMD in Rust, the musl allocator performance issue, and WASM versus native Node module performance data.
AFAIK you don't really get native performance, although it's hard to find good data on this (https://nickb.dev/blog/wasm-and-native-node-module-performance-comparison was one I could find on the fly).
WASM is still virtual machine based and not compiled to a native executable so this is to be expected I guess, it's supposed to be faster than JS and it achieves that goal. Maybe it can be an alternative to Unity, which would be good enough for a lot of games, but I don't know how that compares.
I haven't used SIMD in Rust (or Wasm). Syscalls can be passed into the Wasm env.
https://doc.rust-lang.org/core/arch/wasm32/index.html#simd
https://nickb.dev/blog/authoring-a-simd-enhanced-wasm-library-with-rust/
Could definitely be more effort than it is worth just to speed up compilation.
This is addressed in the article: https://nickb.dev/blog/default-musl-allocator-considered-harmful-to-performance/#musls-new-mallocng