The Stonehenges (and all monsters) are subject to the same movement rules as players.
They happen to move at max speed (-75%) while assisting each other leaving the minimum time for them to travel being 7.5 minutes.
Like Ontario asked, where did you see 6 minutes travel time? Did you vision?
Before day break there were none
And as it broke there was one
The moon, the sun, it goes on'n'on
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:. "Leave you inpeace? ... I will leave you inpieces!" - Darksol