I have no idea why, but it takes my Outlook 5 minutes to open. How do I speed this up?

First it would help to know what version. Second, how big is your inbox? Did you try emptying your Deleted and Junk folders?

If that doesn't help, try running ScanPST to fix problems with your PST file.

Your Outlook might be troubling due to any of the reason: such as oversized data files, large number of mails in mailbox, OST/PST corruption, improper outlook configuration or installation etc.
If there is some corruption or any other issues with the PST file, you can fix them by using ScanPST.exe.

To enhance Outlook performance and for efficient management, try out the steps given in article:
10 Useful Tips For Troubleshooting Some Common MS Outlook Errors ~ Email Data Recovery

It could be a faulty add-in causing the slowness. Try opening Outlook in safe mode, and see if the problem persists. To open Outlook in safe mode hold down the ctrl key as you're opening Outlook. Outlook should pop up a message asking if you want to open it in safe mode. Select yet. Safe mode effectively disables all add-ons and a few other things. If you're not having the issue in safe mode then chances are it's an add-in causing the issue. You'll need to go through and disable each add-in one at a time until the problem stops to find out which one is the cause.