Well wouldn't know it! I was trying to determine the exact lines of code I had uncommented, and upon running the code this morning, I get around 35 fps instead of 5!
I don't know what happened, because I did try it yesterday (following your post), and it was still running awfully slow.
So how good is 35fps? It doesn't seem very high to me for a simple spinning cube, so maybe you could try it out.
Get the C# code from Part III, comment lines 122, 123 and 95, and uncomment lines 98, 101 and 120. You should get a spinning cube.