With the recent plublish of SU artwork, it was revealed that Beach City is located around Rehoboth Bay, and that the Galaxy Warp, the place where the Gems used to teletransport to another colonies from another planets, is located in the "Tunguska Sea", that is located in middle of the real Rusia. I say this cuz in the episode "Same Old World", Lapis along with Steven travelled to "Rusia".
Using this calculator, and the city of Surinda as reference (city is located in middle of Rusia, or close to the supposed Galaxy Warp), the distance between Rehoboth Bay and Surinda is around 8700 km
However, the first time that Lapis and Steven goes to the Galaxy Warp, they were located in Empire City; such place isn't mentioned in the map, but is the expy of NY, so using the same calculator, the distance is around 8500 km
We haven't an exact timeframe, but it was already night when they leave EC, and is morning when they get into the barn, so they go and return in matter of a night, that has a duration of around 8 hours
Finally the calc, speed would be (8700000 + 8500000)/(8*3600) = 597.22 m/s ~ Mach 1.74