There are numerous possible causes for this. You might want to list all of the various (if any) applications (like messenger apps) that you are using with Outlook. Sometimes Outlook waits for a "helper" app like Messenger get logged on before Outlook will finish coming up. Also, configuring Outlook to work with http mail servers can really increase the Outlook startup time if the server is slow to grant a logon. (If the connection to the http mail server, like Hotmail, is timing out, this adds 60 seconds to the Outlook startup time.)
Of course a huge PST file may be a problem, too, but it would have to be really large to cause much of a slowdown on a newer system. If none of the items suggested in the first paragraph pan out you might try removing deadwood in various folders, and you might try compressing the file, too. Remember that Outlook doesn't free up space from deleted items until you shut the app down and restart it.
- prosaic