I've been playing around a bit with JPEG2000 (slowly learning about the format, trying to write a decoder for it) --- whereas JPEG normally uses Huffman compression for the bitstream, which although not really parallelisable is relatively fast (essentially 1 table lookup per output value), AFAIK the bottleneck in JPEG2000 decoding is the arithmetic compression, which can't be parallelised either, and involves quite a few more operations than Huffman's inner loop.