The vehicleDetectorYOLOv2 does not support vehicle detection from a bird's-eye-view. It only supports vehicle detection from cameras mounted inside ground vehicles as shown in this example:
For UAV applications, I suggest training a custom YOLO v2 object detector.
With regards the performance being slow, your object detection function is loading the detector from disk for every frame. Make your detector persistent to ensure that data is only loaded once:
persistent detector;
if isempty(detector)
detector = vehicleDetectorYOLOv2();
end
