As example:
1. View https://phabricator.wikimedia.org/T108263#1574637.
2. Click the first image to open in the lightbox.
3. Navigate to the next images (second and third).
4. Navigate back to the previous images.
The initial opening is slow because the image follows two redirects:
* https://phab.wmfusercontent.org/file/data/msh7../PHID-FILE-c6i../flow-font-175em.png
* https://phabricator.wikimedia.org/file/data/msh7../PHID-FILE-c6i../flow-font-175em.png
* https://phab.wmfusercontent.org/file/data/msh7../PHID-FILE-c6i../psf../flow-font-175em.png (UNIQUE)
Each subsequent navigation is slow for the same reason. However the worst part is that when navigating backwards or re-opening the image a moment later it goes through the same redirects again, and ends up on a different (unique) url. Thus going through several client-server roundtrips every time *and* re-downloading the same file again because it's at a different url.
The end result is that it takes about 1-2 seconds between images which is sufficient cognitive time to lose the mental short-term imprint of the visual and not be able to tell the exact difference between them. Something that makes going through a few dozen design iterations very irritating.
For now I workaround this by opening them in tabs instead, and switching between those.