detecting faces function "cvHaarDetectObjects" took 13s

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I wrote a opencv program to detecting faces from camera, it works well in my Ubuntu

But when I moved this program to the Raspberry Pi, it worked so slow,so I checked the time that every step takes, so I found the function "cvHaarDetectObjects" took 13s !!

So I don't know how to solve this problem. I need help !

Below is the program I wrote

#include "cv.h" 
#include "highgui.h"

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <assert.h> 
#include <math.h> 
#include <float.h> 
#include <limits.h> 
#include <time.h> 
#include <ctype.h>

#ifdef _EiC 
//#define WIN32
#endif

static CvMemStorage* storage = 0; 
static CvHaarClassifierCascade* cascade = 0;

void detect_and_draw( IplImage* image );

const char* cascade_name = 
"/Users/ang/Documents/workspace/eye/src/haarcascade_frontalface_alt.xml";
/*    "haarcascade_profileface.xml";*/

int main( int argc, char** argv ) 
{ 
//   cascade_name = "haarcascade_frontalface_alt2.xml"; 
    cascade = (CvHaarClassifierCascade*)cvLoad( cascade_name, 0, 0, 0 ); 

    if( !cascade ) 
    { 
        fprintf( stderr, "ERROR: Could not load classifier cascade\n" ); 
        return -1; 
    } 
    storage = cvCreateMemStorage(0); 
    cvNamedWindow( "result", 1 ); 
     cvNamedWindow("source",1);
        CvCapture *capture = cvCaptureFromCAM(0);
        if(!capture)
                printf("\n");
        IplImage* image = NULL;
        char key = cvWaitKey(10);
    while( key != 'Q' && key != 'q' ) 
    { 
                image = cvQueryFrame(capture);
                        if(!image)
                        {
                        printf("image NULL");
                        continue;
                        }
                std::cout<<"width"<<image->width<<std::endl;
                std::cout<<"height"<<image->height<<std::endl;
                //cvShowImage("source",image);
        detect_and_draw( image ); 
        key = cvWaitKey(10);
        //cvReleaseImage( &image );   
    }
        cvReleaseImage( &image );  
    cvDestroyWindow("source"); 
        cvDestroyWindow("result"); 

    return 0; 
}


void detect_and_draw(IplImage* img ) 
{ 
    double scale=1.2; 
    static CvScalar colors[] = { 
        {{0,0,255}},{{0,128,255}},{{0,255,255}},{{0,255,0}}, 
        {{255,128,0}},{{255,255,0}},{{255,0,0}},{{255,0,255}} 
    };//Just some pretty colors to draw with

    //Image Preparation 
    // 
    IplImage* gray = cvCreateImage(cvSize(img->width,img->height),8,1); 
    IplImage* small_img=cvCreateImage(cvSize(cvRound(img->width/scale),cvRound(img->height/scale)),8,1); 
    cvCvtColor(img,gray, CV_BGR2GRAY); 
    cvResize(gray, small_img, CV_INTER_LINEAR);

    cvEqualizeHist(small_img,small_img);

    //Detect objects if any 
    // 
    cvClearMemStorage(storage); 
    double t = (double)cvGetTickCount(); 
    CvSeq* objects = cvHaarDetectObjects(small_img, 
                                                                        cascade, 
                                                                        storage, 
                                                                        1.1, 
                                                                        2, 
                                                                        0/*CV_HAAR_DO_CANNY_PRUNING*/, 
                                                                        cvSize(30,30));

    t = (double)cvGetTickCount() - t; 
   //// printf( "detection time = %gms\n", t/((double)cvGetTickFrequency()*1000.) );

    //Loop through found objects and draw boxes around them 
    for(int i=0;i<(objects? objects->total:0);++i) 
    { 
        CvRect* r=(CvRect*)cvGetSeqElem(objects,i); 
        cvRectangle(img, cvPoint(r->x*scale,r->y*scale), cvPoint((r->x+r->width)*scale,(r->y+r->height)*scale), colors[i%8]); 
    } 
    for( int i = 0; i < (objects? objects->total : 0); i++ ) 
    { 
        CvRect* r = (CvRect*)cvGetSeqElem( objects, i ); 
        CvPoint center; 
        int radius; 
        center.x = cvRound((r->x + r->width*0.5)*scale); 
        center.y = cvRound((r->y + r->height*0.5)*scale); 
        radius = cvRound((r->width + r->height)*0.25*scale); 
        cvCircle( img, center, radius, colors[i%8], 3, 8, 0 ); 
    }

    cvShowImage( "result", img ); 
    cvReleaseImage(&gray); 
    cvReleaseImage(&small_img); 
}