Over enemy giant airfield and town at field 88, framerate going from over 60 to 20ish.  I've got almost all details turned down/off except ground detail is maxed, bump mapping, and other plane skins.  Everything else is off.  Video mem usage is ~550mb with an R9 380.
Under any other conditions, I have had no issues with framerate that I have noticed.  Even in FSO with a big bomber group.
Many other reports of people with the same issue in the area.  Widely varying machine specs.  One person with a high end rig and 144hz monitor had it drop to ~60-70.
Wiley.