[londons_explore]

I don't really understand why they would be slower to decode. It's really just the same data ordered differently in the file.

I can see that if you try to render an incomplete file you might end up "wasting" effort blitting it to the screen and stuff before the rest of the data is decoded. But if thats a concern, one can simply rearrange the data back to scanline order and decode as normal?

[robryk]

They are slower to decode mostly due to decreased cache locality. In sequential JPEGs you read one block worth of data, make pixels out of it, and write the pixels out. In progressive encoding, you need to write the pieces of coefficients back to memory at every scan -- the whole image won't fit into cache -- so there's one more memory roundtrip for every pixel. Also, there's just more symbols to decode in total.