Extremely slow Outlook

Hello, my Outlook (XP) is unworkably slow. I'm on Windows XP. I've tried the following solution but the Use Cached Exchange Mode check box was already selected.

In Outlook 2010:
1. Click the File tab, click Account Settings, and then click Account Settings.
2. On the E-mail tab, select the user’s Exchange Server account, click Change, and then make sure that the Use Cached Exchange Mode check box is selected.