When you open Task Manager in POA, you can often see the huge number of tasks in queue 'ADSCTask_*' for a particular subscription.
Most other subscriptions do not have this issue. Customer (owner of the problem subscription) is subscribed to the Directory Synchronization Service (CDI, Customer Directory Integration).
In most cases a huge number of AD related tasks can be explained by changes in subscription. For instance, if a subscription with a lot of users is destroyed, it is normal that a huge number of AD tasks is created to clear these subscription's users from Provider's AD.
If no significant changes are made to the subscription or customer's Active Directory, it is possible that ADSync service is not configured properly on customer's side.
POA Subscriber's Guide contains a step that needs to be executed before initial AD synchronization -
ADSync.exe clear /all, step #9 at the very bottom of the page.
This step clears local cache and ensures that all accounts configured for synchronization will be properly synchronized. However, it is only required for initial synchronization.
If it is executed before every synchronization attempt, rather than sending only changed information the Synchronization Agent will perform complete synchronization of all users each time, creating an unnecessary load.
Check with the customer that they are not running ADSync.exe with the 'clear' parameter in their synchronization scripts.