There is some lift produced by the low pressure in this pocket, but mostly, the low pressure acting on the inclined surface creates drag. Whenever there is a region where the surface of the aircraft is directed backwards and it has low pressure acting on it, then there is pressure drag. Normally, well designed aircraft have very little pressure drag. A third source of drag is related to the process of making lift on the wing. This is called induced drag.
Energy is lost in the process of making lift because of the airflow around the wingtips.
The pressure difference between the upper and lower surface of a wing cannot be maintained near the tip, because the flow leaks around the edge of the wingtip. This leakage creates a tip vortex that trails off behind the wing. You can see this vortex when the plane flies through smoke. There is a drag force associated with the energy lost in the tip vortices, called induced drag. If the lift is spread out over a longer wingspan, the effects of the wingtip are reduced so there is less energy lost in the tip vortices.