Interesting case scenarios:
1) a Site has a stupid big gif that shows logo and staff parties, basically adding nothing but sucking up bandwidth(Not your case)
2) Your case, where you actually add value-content with the gif.
Now the speed metrics are just that, speed metrics they "report" in isolation from "content".
So now my question is: Is Google's OTHER-content-signals good enough to overcome any penalty that might have been applied because of the speed ?
You might be able to turn a 300kB GIF into a much smaller encoded video; as long as it doesn't have audio, you can autoplay it.