The Mac version seems to be prone to these slowdowns.
Try creating a new user account (System Settings>Users & Groups), signing in to that, then testing if the problem still exists. If it goes away, it's an issue with the user profile part of your Office installation.
If the issue still exists, or you'd rather not move your user data to a new profile, try uninstalling Office following these steps: Uninstall Office for Mac Then reinstall from the Subscriptions page of your Microsoft account.