Join an empty server, open up a network monitor.
Have your friends, 1 by 1, log in next to you. watch the revived data increase every time.
We all have a decent amount of loot, so it could potentially be related to an item we are all carrying.
EDIT* Turns out, its caused by items that have an inventory themselves (medkits/protector cases/ammo boxes)
while there's an increase in network activity per client in the vicinity. its drastically reduced as long as nobody is carrying items with an inventory