It varies by the runtime, and the codegen of the WASM itself. From some benchmarks & anecdotes I've seen, the faster runtimes (v8, Spidermonkey, WAVM) are within about 10-50% of native speeds, give or take. There's also some runtimes (like Wasmer & WAVM) which provide the option to AOT compile your WASM module to native. In those cases, the gap from native is much smaller. But so far the JIT for WASM is just immature.

That said, from what I've read, it looks like starting up a new WASM instance is pretty fast, so some places are using it for when they need to spawn up tons of instances all at once, without having to wait for a whole process to warm up.