Over the weekend I deployed a new plugin I wrote for Perlbal that tracks bandwidth usage of certain features on Amie Street. The plugin is only running on 25% of our traffic for now, but after a couple of days I had collected plenty of data to start analyzing it this morning.
The first thing I noticed is that a single one of our images was accounting for 10x as much bandwidth use as the next most transferred image on the site. I loaded it up to find that it was only a 128x128 album thumbnail, yet was a 560KB file. I downloaded it and tried to figure out why it was so big, but without much luck. I soon figured out that I could use convert -scale 1x1 to scale it to a single pixel and it still took over 500KB.
After a bit of googling I came upon jpegoptim, a utility that performs lossless compression on JPEGs by reconstructing the huffman encoding without changing the image contents. Simply running jpegoptim image.jpg I cut the image size down to 28KB and reuploaded it to production.
After scanning the top 1000 largest thumbnail images on the site, I found that the majority of them could be decreased by a good 50% or more by simply using this tool. So, if you've got a site that has a lot of JPEGs (particularly user-uploaded content that's automatically scaled using ImageMagick) give jpegoptim a try. The worst that happens is that it will report that the file is already optimal; the best case is a 95% reduction in load time for users with slow connections.