Cutting VideoCapture latency on USB cameras

If your loop is slower than the camera, VideoCapture hands you stale frames from its internal buffer. On Linux with V4L2 you can shrink it:

cap = cv2.VideoCapture(0, cv2.CAP_V4L2)
cap.set(cv2.CAP_PROP_FOURCC, cv2.VideoWriter_fourcc(*"MJPG"))
cap.set(cv2.CAP_PROP_BUFFERSIZE, 1)

Not every driver honours BUFFERSIZE. The reliable fix is a grabber thread that always keeps only the newest frame.