急！急！急！！！高分：图像显示速度问题

一下代码是当图片大于显示窗口时用鼠标移动图片（象ACDSee那样），但移动时图片显示很慢，不知道是何原因请各位相助.
void CShowJPEGView::OnMouseMove(UINT nFlags, CPoint point)
{
if(m_bBmpMove)
{
CPoint p;
p=m_pntBmp+(point-m_pntLBDown);
m_nX=p.x;
m_nY=p.y;
if(m_nX>0)
m_nX=0;
if(m_nY>0)
m_nY=0;
if(m_nX<m_ClientRect.Width()-m_nWidth)
m_nX=m_ClientRect.Width()-m_nWidth;
if(m_nY<m_ClientRect.Height()-m_nHeight)
m_nY=m_ClientRect.Height()-m_nHeight;
CDC* pDC=GetDC();
pDC->StretchBlt(m_nX,m_nY,m_nWidth,m_nHeight,&m_dcMemory[3],0,0,m_BitmapSize.cx,m_BitmapSize.cy,SRCCOPY);
ReleaseDC(pDC);
m_pntLBDown=point;
m_pntBmp.x=m_nX;
m_pntBmp.y=m_nY;
}
}