The speed of sound is commonly used as “the speed of sound in the atmosphere”, so something around 1200 Km/h.
Either way if a creature like that existed, you’d have to question what we are looking at, it’s either NOT a galaxy in space, maybe a bunch of luminescent pollens or insects, or, if we are indeed in space, this would be definitive proof that there’s some intelligent creator out there and the laws of the universe are clearly arbitrary.
It would still be unclear why such a specific silly event would take place, so I guess we’d have to give much more credit to the simulation theory.
So I guess it’d close its mouth faster than the speed of light.