This is related to Ticket #898031 where Territories Utilities is taking 3-4 days to run if the data volume is close to 1 M. When we raised a Support Ticket, we got to know that the main reason for the slow performance is due to the fact that Mass Assignment does not use Bulk API currently.  Switching to Salesforce Bulk API is considered an enhancement, with no future release targeted. Since there are many records, the performance of the Territory Utilities related process is currently expected, if you run them at the country/market level.

Please consider this for the enhancement to improve the performance.

Regards,

Abhiranjan