kindly see this please
You can use no more than 30 External Live Feed files in these URL category definitions, and each file should contain no more than 5000 entries. Increasing the number of external feed entries causes performance degradation.
from user guide User Guide for AsyncOS 14.5 for Cisco Secure Web Appliance - GD (General Deployment)
under section : Creating and Editing Custom URL Categories
also please advise that, more entries will have more impact on the performance
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