I noticed that Firefox is very unresponsive when incrementally decoding the image from this page on 10.6 Mac nightly 11.0a1 (2011-11-26): http://imgur.com/RX5UI Image info gives: 3387px × 2408px (scaled to 610px × 434px) STR: 1) Go to http://imgur.com/RX5UI 2) Use shift+reload

If you profile this I expect you will see that this is because of the redrawing and not the decoding. I had think I had a patch on some bug to improve this, I don't remember if I landed it though.

This is my most common source of jank. I should note that it's worst on Mac due to the high-quality scaling Quartz does.

A question. Do we end up redrawing the whole image, or just the newly-decoded area?

At least for JPEG, it's supposed to be only the newly-decoded area: http://mxr.mozilla.org/mozilla-central/source/image/decoders/nsJPEGDecoder.cpp#666

Yes; the question is what our drawing code (and perhaps layout invalidation code) actually ends up doing....

Assigning P1 in light of comment 2, but if this is Mac-only maybe this is only a P2?

Assigning to Jeff to push this forward and/or reassign within GFX.