This takes about 4800 milliseconds without the SuspendLayout and ResumeLayout calls, and 1600 with them, so you might try adding suspend/resume calls around your code that creates nodes.

string text = "";
for (int i = 0; i < 100; i++)
	text += "test ";

DateTime start = DateTime.Now;

diagramView.SuspendLayout();
for (int i = 0; i < 100; i++)
{
	TextBox t = new TextBox();
	t.Text = text;
	t.Multiline = true;

	ControlNode node = new ControlNode(diagramView, t);
	node.Bounds = new RectangleF((i % 6) * 30, (i / 6) * 30, 20, 20);
	diagram.Nodes.Add(node);
}
diagramView.ResumeLayout();

MessageBox.Show((DateTime.Now - start).TotalMilliseconds.ToString());
 

If you also have auto-routed links in the diagram, you can call diagram.LinkRouter.Suspend / Resume to prevent link re-routing while creating new nodes.

I hope that helps,
Stoyan

From what I can see, the view is not fully repainted while scrolling and the textboxes leave some traces on it. You could force it to refresh while the scrollbars are being dragged:

(diagramView.HScrollBar as ScrollBar).Scroll += OnScroll;
(diagramView.VScrollBar as ScrollBar).Scroll += OnScroll;

void OnScroll(object sender, ScrollEventArgs e)
{
	diagramView.Refresh();
} 

There's still some flicker, but I think that's the best you can get using ControlNodes, which involve moving controls' windows around when scrolling the view. If you are using them only to display editable text, you could replace ControlNodes with CompositeNodes that contain EditComponents, or simple ShapeNodes that invoke BeginEdit from the NodeClicked handler.

I hope that helps,
Stoyan