But what has really struck me about the mobile experience is that using HTML5 video significantly improves the performance. This is important for GIFs/videos with a high resolution, which as GIFs would not render in real time on my phone (which has an S800, so I can only imagine what the experience must be like on other phones).
Video is video and is in general more properly streamed, evicted from memory, and halted when offscreen.
"... But in the worst case all your elements might be grouped into a single layer and the browser has to repaint every single element. And, when it’s done, it still needs to upload everything to the GPU. All of this is work occurs for every GIF frame..."
GIF's are really a nineties format (90ties??), the animation and alpha components weren't added to the standard until 1989, and adoption wasn't widespread until after then.
You're also overlooking the fact that you simply didn't get 4.5MB+ GIFs in the nineties like the example given because it would take ten minutes to download that on a 56K modem. I must resign myself to the fact some people never grew up with dial-up!
Animated gifs in 1995 were rather different to animated gifs now.