Caching mailboxes that are either larger than 2GB in size or contain more than 5,000 items can negatively affect Outlook performance. Instead of deleting old emails or archiving them to PST, if this is not an option, you can set a filter on a particular folder so that it is not stored in the OST cache, but is still available when connected to the Exchange folder (and backed up).

1. Right click on the folder and select Properties
2. In the Synchronization tab, click on the Filter button
3. Go to the More Choices tab
4. Set "Size (kilobytes) less than 0"