Figure 1: Example #1 of slowing down animation speed in the middle of a sequence, in this case the trajectory of a ball (the AMIGA logo). If you cannot see the full animation, please reload the page.
Figure 2: Example #2 of slowing down animation speed in the middle of a sequence, in this case a semaphore embedded in two circles. If you cannot see the full animation, please reload the page.
And now, Art Basel Miami Beach is hosting an animated .gif exhibit/competition called "Moving the Still" :
Exhibits will be posted to the Tumblr site as they are received. Michael Stipe (of REM fame) is one of the organizers. Should be lots of fun.NOTES:
 the entryway to my website is (and has been for about 12 years) fronted with various animated .gifs.
 the .gif format uses LZW lossless compression, which keeps the file size small and preserves most color variation in source images (some features, however, may not be preserved).
 technically, this would be called an animated anaglyphs. I use the software Anaglyph Maker to properly align regular .bmp images. The depth-illusory .bmps then served as input for the animated .gif. An example can be seen here:
BONUS: here is an excellent video on the history of Animated .gifs from the PBS show "Off Book".