From my understanding, most of the WASM performance improvement expectations have been around the cost of calling browser APIs from within WASM. The performance itself is basically near native speed with a small overhead for the cost of bounds checking/wrapping the memory block. Last I checked, most WASM apps are calling DOM APIs through a javascript middleman, which obviously sucks for performance. But native importing of DOM APIs is something that I believe was being worked on and could be here soon?