这个是我要操作的图片，需要您把picturebox添加到一个form窗体中然后绑定以下事件

public Form2()
{
InitializeComponent();
}
Bitmap Ima = new Bitmap("b.jpg");
private void Form2_Load(object sender, EventArgs e)
{
pictureBox1.SizeMode = PictureBoxSizeMode.Zoom;

        pictureBox1.Image = Ima; pictureBox1.Location = new Point(0, 0);
         pictureBox1.Size = new Size(14000, 800); 
    }
    Point p1 = new Point(); Point p2 = new Point(); bool status = false;
     
    private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            p1 = new Point(e.X, e.Y); status = true;
        }
    }
    Rectangle rec = new Rectangle();
    private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        Point p2 = new Point(e.X, e.Y);
        if (p2.X < 0)
        {
            p2.X = 0;
        }
        if (p2.Y < 0)
        {
            p2.Y = 0;
        }
        if (p2.X > pictureBox1.Width)
        {
            p2.X = pictureBox1.Width;
        }
        if (p2.Y > pictureBox1.Height)
        {
            p2.Y = pictureBox1.Height;
        }
        if (p2.X > p1.X)
        {
            rec.X = p1.X;
            rec.Width = p2.X - p1.X;
        }
        else
        {
            rec.X = p2.X;
            rec.Width = p1.X - p2.X;
        }
        if (p2.Y > p1.Y)
        {
            rec.Y = p1.Y;
            rec.Height = p2.Y - p1.Y;
        }
        else
        {
            rec.Y = p2.Y; rec.Height = p1.Y - p2.Y;
        }
        this.Refresh();
    }

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        if (status)
        {
            Graphics gg = e.Graphics;
            gg.DrawRectangle(new Pen(Color.Red, 2), rec);
        }
    }

以上是我的代码，出现重绘缓慢的是鼠标左键在控件中点击以下 然后拖动绘制矩形很慢跟不上鼠标的速度，我切换其它图片就没有问题，并且picturebox大小变成20000*800不存在这个问题