I have some moveable and resize-able controls. If I move it while it’s on top of another control, it is smooth and good. When I drag it over the form background, the movement suddenly lags a lot. What could be causing this? video link (the background is the gaps between the controls)

My controls are all double buffered

protected override CreateParams CreateParams
{
    get {
        CreateParams cp = base.CreateParams;
        cp.ExStyle |= 0x02000000;  // Turn on WS_EX_COMPOSITED
        return cp;
    }
}

The resize-able control is a custom control inheriting from Panel

class ResizeablePanel : Panel
{
    public ResizeablePanel()
    {
        
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        if (this.Parent.GetType() == typeof(SplitterPanel))
            return;
        Rectangle rc = new Rectangle(this.ClientSize.Width - grab, this.ClientSize.Height - grab, grab, grab);
        ControlPaint.DrawSizeGrip(e.Graphics, this.BackColor, rc);
    }
    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);
        if (m.Msg == 0x84) {  // Trap WM_NCHITTEST
            if (this.Parent.GetType() == typeof(SplitterPanel))
                return;
            Point pos = this.PointToClient(new Point(m.LParam.ToInt32()));
            if (pos.X >= this.ClientSize.Width - grab && pos.Y >= this.ClientSize.Height - grab)
                m.Result = new IntPtr(17);  // HT_BOTTOMRIGHT
        }
    }
    private const int grab = 16;
}