During the upgrade we do a lot of modification of data dictionary, this is the reason why the upgrade script will collect dictionary stats at the end. The stats collection procedure will collect stats only for stale tables. If your database had a lot of stale tables before upgrade then upgrade script will run longer.
To decrease the amount of downtime incurred when collecting statistics, collection of statistics is suggested, prior to performing the actual database upgrade. You can also check how many stale objects you have before the upgrade and based on the output, decide collect stats before the upgrade or not .
Script To Check Schemas With Stale Statistics [ID 560336.1]