Since all languages should be able to compile to wasm, that question is pretty close to "Which language is the most performant?", which is a can of worms that I'm not going to open. Maybe something like the Benchmarks Game would be useful here. Or already exists, and someone could point me to it :)

A similar question would be "What languages do you know of that are focused on performance and compiling to wasm?" For that, I'd say C++, Rust, and Go, though I'm sure there are others.

It's really hard to say, especially as the wasm interpreter stuff itself is under so much flux; different implementations act differently, etc.

There's also the question of what aspect of performance is of interest to you. Do you include the download time in performance? If so, languages that produce leaner wasm files will be better performing, even if the execution is identical. Do you only care about execution time? Then that aspect isn't relevant. etc.

That depends on (a) how you define your measure for "performance" (see other comments that talk about the difference between microbenchmarking and application benchmarking), (b) the compiler that you use, (c) the WebAssembly implementation that the code finally runs on.

So, do not expect a clear answer to that questions. Probably no one really knows because of a lack of benchmarks and deployments in the wild that have been analyzed. That's why they made the benchmark.

If you really do mean fastest and nothing else, you can't really beat WAT, the WebAssembly text format.