I am not the op, but (faster) WebAssembly could turn out to be a big thing for browser extensions. For example, we are just in the process of adding WebAssembly powered image search to our Kantu Selenium IDE to allow for visual testing. We tried before with normal Javascript and even WebGL, but both results were not usable for various reasons. The websight page gave us new hope ;-) https://websightjs.com/
Faster WebAssembly isn't listed on the roadmap, so you are disappointed, right?
Wasm is currently considered "around half as fast as native"[0]. The Wasm design, Emscripten compiler, and browser compilers all have a part of responsibility in this, but I suspect that browser compilers have the largest share. Mozilla created The WebAssembly Explorer[1] which shows good-looking ".wat", but bloated "Firefox x86 Assembly" compared to "LLVM x86 Assembly". I hope they intend to use this tool to improve Firefox x86 Assembly.
How many websites do you visit (and which ones are they) that use WebAssembly?