Webassembly looks like an upcoming standard for sandboxing efficient code on the server side as well. As it's well supported and it doesn't have a big startup runtime cost, it looks like it has a real chance of success right now (after many failed ones in the last 25 years).
SIMD support is being worked on as far as I know, and this is a problem where there is room for a lot of improvement in the future. Generally interfacing between programming languages and systems efficiently, without too much data conversion/memory management is a harder problem than adding more low level programming support.
It's kinda lame that none of those demos even run at 60fps, let alone 120fps on mobile for a real smooth experience, with lots of performance headroom for more than a single feature at once...
There's no reason to think it won't help - the encoder/decoder code base could be written in WASM directly to ensure it uses the SIMD instructions, and those will hopefully map closely to the machine SIMD instructions.
Of course though, you're right that it'll still incur a performance penalty.
Someone needs to make a dev environment where someone can write WASM and see realtime the generated assembly instructions, together with the number of cycles to execute.
That way, the developer can tweak the input to the compiler to get exactly the sequence of instructions they wanted.
They can also hand write the output assembly, and put a patch in to the compiler saying "this generated assembly is faster that what you generated, so please generate this in the next version".
