Hi,
Auto upgrade should not slow the network. SEPM won't upgrade all the clients at a same time.
It would upgrade only 10 clients at a time. Also you can schedule upgrade time during non production hours.
Have you tried to check performance after stopping auto upgrade?
If you have limited number of client then use CDW method & deploy new package during non production hours.
As everyone said always follow the best practice
Best practices for upgrading to Symantec Endpoint Protection 12.1.2
http://www.symantec.com/business/support/index?page=content&id=TECH163700