In a browser you only get one thread, the JS thread. If you use a web worker, you can have one additional thread, that work in parallel to your UI thread.

Wasm (web assembly) is a standard that let you run binary code on your machine, this code can run close to native speed and use less memory.

The goal here is to free the JS thread as much as possible so that it is non blocking and smooth.

All code that is not related to the UI run at native speed in parallel.