I've experienced this under Firefox, and on hardware that has no excuse for breaking a sweat on any webpage. It's usually pages where there are a large number of animated GIFs (e.g. [1]) Once all the GIFs are loaded and complete one animation cycle, everything's fine.

[1]: http://www.totalprosports.com/2012/12/14/the-50-best-sports-...

Are you sure you're not referring to animated GIFs? The slow FPS rate occurs as they load, and is contained to the GIF itself. Surely a static GIF would not reduce your browser FPS.

Don't be so sure it's a loading issue. I very often have animated gifs play very laggily the first time they run, even when they were already fully downloaded. And across multiple browsers I have seen issues playing gifs smoothly when there is anything else in the browser using CPU.