Windows 7: Severe Outlook 2007 Performance Issues

I came across a weird one yesterday, and it's been resolved, but I'm curious as to what was the cause of the issue.

One of my user's was complaining about severe slow downs in Outlook 2007. It would take minutes to open an e-mail, and Outlook would often lock up for 30 seconds to a minute as well. It's an Exchange account, running in Cached Exchange Mode. So, I deleted her profile and recreated it. That didn't work. So, I created an new hard drive for her using my known good image. I added in her account, and Outlook was running beautifully.

She came in to my office yesterday with her laptop. I backed up her data, swapped drives, and restored everything back to her laptop. Still, Outlook ran perfectly. As she was about to leave, I remembered that I still needed to restore her Outlook and Signature folders (to her AppData folder). I do this so their auto-complete file gets restores (.nk2 file).

Once I copied those small folders back, Outlook started to run like molasses. I renamed the Outlook folder to Outlook.old and started Outlook up again. A new Outlook folder was created, and the program ran perfectly. I copied back just the .nk2 file, and it still ran perfectly, but her autocomplete worked as well.

In the end, it was a simple fix, and the user was happy, but I'm left wondering what in this folder would cause such issues? In my folder, I have an .xml file, my .nk2 file, and several .srs files.