If you add extra Pouches (even empty ones) or a pistol holster (also empty one) to the Plate Carrier, the Stamina indicator decreases abnormally strongly, by about as much as a full Mountain Backpack takes.

ALSO

If you add Handheld Transceiver to your Backpack, the Stamina indicator decreases abnormally strongly, although the Transceiver weighs only 2 kilograms (which is a bit too much for this device, but still). A SMG with ammo (3kg) in backpack reduces mobility less.