IT Administrators are familiar with "Outlook seems sluggish," "Why does it take so long to sort items in my inbox? issues." And, the culprit may be too many items in a folder. "For users running Office Outlook in cached mode, problem generally manifests itself in sluggishness of client, as Outlook sifts through large amounts of data, resulting in a dissatisfactory e-mail experience for users.
So how many items are too many items in a folder? from Exchange best practice point of view, you don't want to see more than 20,000 messages in single folder. And that's particularly true for critical path folder, including "Inbox, Contacts, Deleted Items, Calendar, or Sent Items folders".
To reduce this item count, the easiest solution is to enable "Auto Archive" in Outlook. You also might want to implement a mail retention policy for your organization, which'll auto delete items older than the limit in the policy.
More Info: AutoArchive feature in Outlook 2007 and in Outlook 2003 (KB830119)