Just a couple of ideas: 1) Call BeginUpdate before you add the nodes and then EndUpdate once you are done. 2) If possible, use the Nodes.AddRange method instead of the Nodes.Add method. Regardless, half a million lines in a treeview sounds a tad daft...

What about using the Threads window and freezing/thawing threads as you need them?

Could you not use a MemoryStream and Image.FromStream?