The Adaptive Images plugin seems to do it for mobile.
Changing the resolutions setting to: 3840, 2560, 1920, 1280, 1024, 640, 480 makes it work for desktop as well.
Since adding https on the blog, my PC struggled to browse the galleries due to having to decrypt all those enormous images; after installing the plugin, I can even browse it on my phone without problems.
The CPU of that instance is hard at work resizing images as you can see and the cache directory is rapidly filling up with resized images, yay!
