According to Wikipedia WebAssembly may get about 65% of native assembly speed (mileage may vary).
WebAssembly runs on a VM and is more a 32-bit virtual machine running on modern hardware as much as Steve Wozniak’s Sweet-16 ran on the Apple 2 series (could run on other machines easily).
The biggest “value” for running WebAssembly on another processor rather than native is a portable executable that doesn’t need to be recompiled natively (or, not as much) but you lose so much more in the process of that, including execution speed, energy efficiency, and you need to have a lot of other things added to make it useful.
It’d be interesting to see how efficiently a Mill processor can execute the VM in practice because it’s clear it’s something a number of online things will use in web browsers. Outside of web browsers it makes far more sense to compile a higher-level language down to local Mill ISA than to use WebAssembly.