Image metadata can optionally be stripped on transformations, see e.g. http://www.imagemagick.org/script/command-line-options.php#strip (other utilities such as jpegoptim are more fine-grained).
JPEG optimizers (lossless) shows we can win at least 17KB for images on an average Wikipedia page.
- Relevant source code: MediaWiki / Bitmap::transformImageMagick() https://github.com/wikimedia/mediawiki/blob/96ca8551b232a0bc497632f71f7ab083595acbf7/includes/media/Bitmap.php#L70-L79
This comes at the cost of various pieces of meta data.
I propose we should compress image thumbnails and use a single meta data field pointing to the canonical url of the image where all information about the image can be found.
This will yield huge performance benefits for all or users viewing Wikipedia articles.