It's nice to see how much faster is wasm. This kind of problem would also be nice for parallelization. What's the state of multi-threading wasm? any advantage over javascript web workers?
Atomics have universal support in modern browsers.
This is primarily a single-threaded problem. Each x and y update depends directly on the previous x and y values with the x and y being used to update pixels.
At most (barring changes to the algorithm), it looks like you could calculate these on one thread and update the image on another thread.