I'm curious about the performance tradeoffs of Javascript versus native code?

A few weeks ago, I tried writing something that was long-running, CPU intense, ect, in a webworker. It was so darn slow that I switched to a native language. (I hope I didn't do something silly that made my code run more slowly than it should.)

I see some mention about running in WASM. Does this do something like have ordinary Tesseract compiled for WebAssembly and then fallback to Javascript?