Small little nuance found today. If you accidentally paste a large amount of copied text into the search field (got a little trigger happy like I did, CMD+F, CMD+V), you will get the spinning rainbow ball.
The way around this is to:
1) Force Quit iTerm2 (I use Alfred for this)
2) Restart iTerm2 (don't press CMD+F)
3) Select a small amount of text in the terminal screen and press CMD+E
What will happen it the highlighted text with the offending search criteria.