I'm told anti servo tabs are more aerodynamically efficieent, since they create less drag by following in trail w/ the control surface. But then in a trimmed state, say 20 degrees pitch up, what keeps the stabilator deflected if the anti servo tab is in line/flush with it? Is the anti servo tab always moving so quickly , but once it stabilizes the stabilator, it moves to the intrail position again, until a force tries to destabalize the surface again?

More aerodynamically efficient than what?

And no, your typical anti-servo tab is not in line with the control surface if the control surface is deflected.