Performance problems are tricky to diagnose, but you can help by providing a sample. This is also useful if you get a beachball.
For Mac OS X 10.9 and later
- Open activity monitor (in /Applications/Utilities)
- Select iTerm2 in the list of processes
- Click the "i" button
- Click the "Sample" button
- Save the result and attach it to the issue.
For Mac OS X 10.8 and earlier
- Open /Applications/Utilities/Activity Monitor
- Select iTerm
- Press "Sample process"
- Save the file and attach it to a [http://code.google.com/p/iterm2/issues/entry new issue] or existing issue.