On your policy, is it set to scan network drives? Besides, how are you performing the copy operation? Are you dragging and dropping files or mapping a drive first and then copying to the the network drive? Also, please repeat Test 3, with AP enabled, OAS disabled. We can then narrow it down to OAS policy settings. Is Scan Files setting enabled for "On Network Drives"? It should be under OAS properties dialog under "scan items" tab.
Also try Virusscan 8.8 patch 9 and report back.
There's little point in wasting time trying to debug anything earlier than the latest update, though it would be interesting to see with which patch the performance degradation started.
Thanks for you time. I have raised SR with McAfee to look in this.