You mean for accessibility? Maybe but there shouldn't be any fundamental reason why an accessibility tool can read a windows app and not web app, since under the hood Chrome and Firefox can use the same interface this tool interfaces with.

If you meant performance in terms of runtime, I'm not convinced this is the case. Obviously WebAssembly will have some overhead, but I don't think the overhead is critical enough for most applications to matter. Especially if those applications are written in anything other than C/C++, they'll have similar performance characteristics.

You're right, it might be only so slightly worse as to not matter, or be better than a language that is close to the metal, but with worse design.

Obviously, for such a wide prediction I can only speak theoretically...