We use an agent-less scanner (McAfee Move) but I do understand that you cannot go that route.
How many CPU's do you have? What happens if you try to add some more CPU's? You will need at least 2 vCPU's, otherwise Appvolumes and Symantec are both fighting for resources during logon.
And what exclusions do you have? Did you try adding the c:\snapvolumestemp\Mountpoints as an exclusion??