OpenCV 读取摄像头画面卡顿问题

//画面若是分辨率比较高，须要将帧图片进行压缩后再进行识别
resize(gray, graysmall, Size(gray.cols / 3, gray.rows / 3));

cascade.detectMultiScale(graysmall, rects, 1.1, 3, 0 | CASCADE_SCALE_IMAGE , Size(30,30)); 

for (auto it = rects.begin(); it != rects.end();it++)
{
    rectangle(img, Point((*it).x * 3 - 2, (*it).y * 3  - 2),
				Point((*it).x * 3 + (*it).width * 3 - 5, (*it).y * 3 + (*it).height * 3),
				Scalar(0, 255, 0), 2);
}