c#GDI+绘图速度的一些差异不理解

现象：各种图的速度绘制的差异。

1：绘制内存位图，例如

Bitmap _SourceBitmap=new Bitmap(800,480);

Bitmap _DesBitmap=new Bitmap(800,480);

           Rectangle ScreenRect = new Rectangle(0, 0, 800, 480);
            Rectangle rect_1 = new Rectangle(0, 0, 800, 480);

            Graphics gScreen = this.CreateGraphics();

            Bitmap _membitmap = new Bitmap(800, 480);
            Graphics g_membitmap = Graphics.FromImage(_membitmap);

            uint time1 = GetTickCount();
            int nCount = 0;
            for (int width = m_CartoonLen; width <= 800; width += m_CartoonLen)
            {
                nCount++;

                int _width = width;
                int _heihg = width * 3 / 5;
                int x = (800 - width) / 2;
                int y = (480 - _heihg) / 2;

                rect_1.X = x;
                rect_1.Y = y;
                rect_1.Width = _width;
                rect_1.Height = _heihg;

                //恢复矩形区域
                g_membitmap.DrawImage(_SourceBitmap, 0, 0);
                g_membitmap.DrawImage(_DesBitmap, rect_1, ScreenRect, GraphicsUnit.Pixel);
                gScreen.DrawImage(_membitmap, 0, 0);
            }

            uint time2 = GetTickCount();
            uint len = time2 - time1;

.........(省略)

得到的结果是.303

按照5/0.303=15帧的速度

如果是：

_SourceBitmap = TestWinceSpeed.Properties.Resources.WallPaper__0_;
_DesBitmap = TestWinceSpeed.Properties.Resources.WallPaper__1_;

得到的结果是：

257

按照5/0.25=20帧的速度。

说明差异在30%,是否非常明显的。但是不知道为什么。

再做一下试验：

Graphics gScreen = this.CreateGraphics();
            uint time1 = GetTickCount();
            int nCount = 0;
            for (int i = 0; i < 100; i++)
            {
                nCount++;
                gScreen.DrawImage(_SourceBitmap, m_Rect_1, m_Rect_1, GraphicsUnit.Pixel);
            }
            uint time2 = GetTickCount();
            uint len = time2 - time1;

            Console.WriteLine(nCount.ToString() + "Fun Cost Time ms:" + len.ToString());

            gScreen.Dispose();
            gScreen = null;