Actually, I tried disconnecting the ListView from the ImageList by
setting the ListView's LargeImageList property to null, but to improve
performance I had to remove the connection from the designer as well.
Now the adding of images is very fast, but as soon as I connect the
ListView the computer really goes to work..

This example proves my point. It's a simple form with a Button,
ImageList and ListView. It's Part 2 that locks up the CPU for a good
minute or so.

private void button1_Click(object sender, EventArgs e)
       {

           // Part 1, add images to imagelist
           imageList1.Images.Clear();
           label1.Text = "Adding..";
           label1.Update();
           for (int i = 0; i < 5000; i++)
           {
               imageList1.Images.Add(new Bitmap(1, 1));
           }

           // Part 2, connect listview to imagelist
           label1.Text = "Connecting listview..";
           label1.Update();
           listView1.LargeImageList = imageList1;
           label1.Text = "Finished.";
           label1.Update();

       }

Cheers,
Johan Sjöström
MSc, MCAD, MCTS, MCITP