There is indeed an issue with the JPEG format that makes parallelization difficult: the lack of a 'table of contents' with offsets to tiles. Various workaround

There is indeed an issue with the JPEG format that makes parallelization difficult: the lack of a 'table of contents' with offsets to tiles. Various workarounds (for example RST markers or self-sychronizing properties of Huffman) have been proposed, but these are not great and do not work for all images. JPEG XL ensures this information is always available.