Besides the fact that only some image viewing programs and image editors support JXL... it is worth remembering that AVIF is just the image format used within the AV1 video codec. So, when AV1 becomes more popular and starts really showing up in silicon (which Google is putting huge pressure on silicon vendors for), almost everyone will have a hardware AVIF decoder for free, should they choose to use it. If that happens, any decoding speed advantages that JXL has could be blown apart very quickly. This makes AVIF, in my view, a much stronger long-term play, as how many people are going to implement hardware JXL decoders? AVIF will be much faster, perhaps slightly less efficient, and plenty good enough.