We’ve been following Yahoo!’s Best Practices for Speeding Up Your Web Site for quite a while now and have been fine in following some of the tips — that we’ve forgotten to extend out our antennas to hear about what’s going on out there in the world of website page load performance.
This one isn’t really that technical but a “perceived performance” — it’s making your JPEGs progressive (basically if it larger than 10k in file size).
There is also an interlaced “Progressive JPEG” format, in which data is compressed in multiple passes of progressively higher detail. This is ideal for large images that will be displayed while downloading over a slow connection, allowing a reasonable preview after receiving only a portion of the data.
Instead of waiting for that JPEG image load line per line, what you would see at first is a pixelated version of the image in full size and it improves its quality as it downloads.
Read more about it in Progressive jpegs: a new best practice by Ann Robson