I have been dealing with the same problem as well and it seem to be resolved with one of the recent dat files 7022-7025. I read the release notes for all of them and nothing stands out so I was hoping to find the anwser on here as to what was done to resolved the issue as this has been going on for 6months on thousands of PC's. Now all of a sudden it's fixed with no explanation. Based on my situation I had it narrowed down to a .cab file that VSE 8.8 was timing out while scanning. Not sure if you ever found a resolution or if it was magically fixed for you over the weekend as well. I'm curious to know.
If you get the performance issues again, please run the McAfee Profiler to get more information about what the OAS is scanning at that time.