I've seen some rather atrocious techniques employed to bake ads into source but that can still be done without any performance impact. Making that work in WASM would require dynamic recompilation, doable perhaps for smaller apps but for behemoths like Gmail and Docs I couldn't see even Google getting that to scale. I think we'll still have browsers performing additional HTTP requests for that stuff, at least in the short to medium term. So traditional adblocking techniques will still be viable.