Change behavior to retrieve contacts in bulk
In a large scale deployment, or in an undersized system retrieving AF Contacts, or accessing the subscription tab on a Notification Rule can be slow. A solution is of course to properly size the system or reduce the scale, however there is a compounding effect where contacts are retrieved one at a time instead of in bulk. Changing this behavior would significantly mitigate this issue even if the system is undersized as the call would only happen once not once per contact.
This is also the case if there is a performance issue (network, etc...) when accessing AD. As there are multiple single calls, the performance issue is exacerbated.