You may need more ram or memory for your computer. What version of outlook are you using?
Outlook and Office requires a lot of RAM. If your computer has 1GB or less, definitely increase the memory for this system.
Ofcourse, we may require to improve the RAM size. But at the same time we need remove Most of the Junk Mails and Unwanted mails from the inbox. Better to improve its performance, do set Rules and Alerts for your Inbox.