Why is cv::resize so slow?

I'm doing some edge detection on a live video feed:

- (void)processImage:(Mat&)image;
{
        cv::resize(image, smallImage, cv::Size(288,352), 0, 0, CV_INTER_CUBIC);
        edgeDetection(smallImage);
        cv::resize(smallImage, image, image.size(), 0, 0, CV_INTER_LINEAR);
}

edgeDetection does some fairly heavy lifting, and was running at quite a low framerate with the video frame size of 1280x720. Adding in the resize calls dramatically decreased the framerate, quite the reverse of what I was expecting. Is this just because a resize operation is slow, or becuase I'm doing something wrong?

smallImage is declared in the header thus:

@interface CameraController : UIViewController
<CvVideoCameraDelegate>
{
    Mat smallImage;
}

There is no initialisation of it, and it works ok.