It is probably because your .PST file is so big. The .PST file is where Outlook stores everything - email messages, tasks, calendar appointments etc.
First, do a clear out. Delete all messages from your Inbox and any other boxes that you don't need any more. Go to the Sent Items folder and again, delete anything you don't need. Repeat this process for tasks, calendar etc. After you're done all that, empty the Deleted Items folder (acts like Recycle Bin).
But you're not done yet. You've deleted and then deleted - but everything is still in the .PST file. Click on the File menu, then choose Data File Management... Press the Settings button and then press Compact Now. It may take awhile. When its done, everything in your .PST file that you deleted really is deleted. Your .PST file should be much smaller and more manageable, and Outlook should perform much better.