When the Notification Server is set up to manage tasks for hundreds (or thousands) of client machines, the tasks don't process in a timely manner, and performance in other areas may also be affected.
This article provides guidance on how to set the "Maximum Computers to manage per Task Server" for the Notification Server.
ITMS 8.x
If the Notification Server is set up to manage tasks for more than 100 computers, performance will be affected.