Windows 7 slow at opening media files
I downgraded my drivers from 364.52 to 362.00 since I already knew that the 364 drivers were known troublemakers, even destroying peoples cards at 364.72... (regression or deliberate...) it's not surprising that earlier 364 versions would also have had problems.
In other words, I'm fairly certain this was a driver bug. I'm testing now with 364.51
Update: Not seeing this problem on 364.51 but I am seeing some slowdowns in other areas, I think I'll stick with 362 for now.