I think I have resolved this problem. I simply added upgrade.bitdefender.com on the Exceptions tab for the ISA provider.
Performance seems to be faster now on the BitDefender clients, but I'll wait to hear from the employees when they return to work later today. The sluggishness seems to have been at login, when BitDefender downloads its large virus definition file. The client computers may have appeared to hang while avast! scanned the BitDefender file at the server.
Ken