It would require a complete rewrite; I commented previously [0] describing how WebAssembly seems inappropriate for Doppio, and also discussed similar thoughts about SharedArrayBuffer [1]. Also, the main source of slowness is due to the fact that Doppio uses a JVM interpreter and does not JIT; it could be much faster than it currently is with additional engineering.
[0] https://news.ycombinator.com/item?id=11656373
[1] https://news.ycombinator.com/item?id=11655922