I might be wrong, but I think that one should be able to utilize JavaScript engine within native iOS app which will use fast implementation.
SFSafariViewController does support WASM by executing in a separate more privileged process but interaction with the app is very limited.
If JSCore allowed WASM JIT-ing, one could easily port WASI to iOS for full system access, and update iOS apps by downloading new wasm versions. Of course Apple won't do that citing __Insert__App__Store__excuse__here, but everyone knows it's because they want that sweet 30% cut, and that can only be effectively enforced with a tight chokehold on updates