But you are completely right pointing out that there's still some room to improve, specially when compared to running PHP natively.
Right now there's some price to pay for the isolation and sandboxing, but we are working hard to reduce the gap to zero. Stay tuned for more updates on this front!
FrankenPHP is also really good and probably a much smarter play than trying to get your code running under WebAssembly.
The latter will always be faster.
The other caveats about "edge" throughout this discussion aside, if I needed to do this, I'd try to write something in Zig or (gag) JS or something else that compiles to WASM directly rather than writing a script for an interpreter that runs under WASM.
You can deploy apps on the Wasmer Edge cloud, and also host things yourself if you want to, though in the later case the setup will be non-trivial.