I tested this on Nexus 7 / PA4. When topbar is being dragged starting the gesture from inside of it, it moves right away, but if the gesture originates from the outside of it, there is a delay.

To test it, pull the topbar all the way down - it doesn't reach the lower edge leaving some background space. Drag it back up starting from inside the topbar (i.e. placing your finger on the topbar before starting moving up) - it moves right way. Pull down again, and then drag it up starting from outside of it (i.e. placing your finger in that area between the topbar and the lower edge and then moving up) - you'll get a visible delay. Make these movements fast to see the contrast between the two cases better.

Reproducible: Always

Steps to Reproduce:
1. Pull the topbar all the way down.
2. Pull it back up putting the finger on the area below (not on the topbar itself.) it and moving it up.
3. Pull the topbar all the way down again.
4. Pull it back up putting the finger on the topbar itself and moving it up.
Actual Results:  
1. Topbar moves down right away.
2. There is a delay before the topbar moves up.
3. Topbar moves down right away.
4. Topbar moves up right away.

Expected Results:  
All movements should happen right away without any delays.