Have you thought about compiling dcraw to javascript with emscripten? I'm not sure what the performance would be like, but I've seen this work in the past.
I doubt that very much. Raw processing is very fast these days and I don't think you could make it faster with the GPU given the memory copying overhead. Look at darktable for an example of how the GPU is used intensively for the pipeline but the raw processing is all done by the CPU and is very fast.