A few comments are mentioning the implementation is probably dated.
Does anyone have benchmarks that tell a different story?
Not a benchmark, but my 8-bit emulator let's you compare asm.js and wasm performance between browsers and platforms somewhat, and (if compiled natively) also for native x64 or ARM performance:
http://floooh.github.io/virtualkc/
It's implemented in a C-with-classes style C++ though, not Rust.
When you press the 'UI' button, there's a little millisecond timer in the top-right corner which tells you how much time is spent in the 'emulator frame', which is pure asm.js/wasm code which doesn't call out into JS APIs.
You can switch between the asm.js and wasm version through the hamburger-menu-icon at the top-left corner.
I'm seeing the "1.2..1.5x slower than native" performance there.
Some of my other demos are also fairly CPU heavy, for instance the two Bullet-Physics demos here:
http://floooh.github.io/oryol-samples/
These demos have asm.js, wasm and pnacl versions and also have fairly detailed timing information which makes it possible to compare performance to the natively compiled versions.
I do currently see some mysterious performance differences between Mac and Windows though, not sure what's up with that (e.g. my mid-2014 MBP with a 2.8 GHz i5 is giving better performance than the 3.5 GHz i7-4770K in my work PC).
Great demos! What is the UI you're using in the mod and sid players?