Does the gravity wells differ enough between galaxies to have different speeds in space time? How much slower in space time would the biggest galaxy have compared to the Milky Way?
Does the gravity wells differ enough between galaxies to have different speeds in space time ?
You are using these terms in a very confused way.
The gravitational field of an object depends on it's mass distribution. It is normally described as a potential field.
A gravitational field does not a have a speed that characterizes it, not do they have a speed through space-time (the field always propagates at the speed of light, regardless of source).
How much slower in space time would the biggest galaxy have compared to the Milky Way?
Keeping in mind that size has nothing to do with speed, note that are normally two aspects to the speed of an object relative to something else (important to remember that speeds are always relative to some other object that "defines" what is at rest and that this choice is arbitrary - that's the theory of relativity for you).
Galaxies can have a local speed due to e.g. motion within the local group like our own local group of galaxies. This aspect of motion is really just governed to "normal" gravitational effects.
On a much larger scale they can have an apparent motion due to the expansion of the universe. This part of motion is due to a large scale effect only explained by using general relativity and which only has an observable effect over cosmological distances.
The gravitational field of an object has no effect on it's relative motion due to the expansion of space-time, whereas it's gravitational field does affect the relative motion of objects local to it and hence their relative motion to that object.