Assuming that the system is online and in use, you may want to consider doing analyze first to ensure stats are proper, and then vacuum all less-used tables that should be quick then larger/more active tables with the verbose option to see details of why it might be taking time. If you are seeing multiple index scan steps, then maintenance_work_mem is lower than ideal. If you are using default vacuum settings, there is no I/O throttling as I understand the vacuum_cost_delay and vacuum_cost_limit settings. What is your concern with it taking time? Depending how you did the upgrade, there may be no (new/special) need to do the vacuum at all also.
