The various sections of the Admin Dashboard are not real-time. Today's ticket trends and Group Performance would be updated once every 30 minutes ( based on your helpdesk time zone ). The other reporting metrics for the Helpdesk would be updated 10 minutes.
For the Group Performance section, there is an upper limit of 50 groups that could be displayed. If the number of groups is more than 50, the top 50 groups based on ticket volume would be listed first.