In my experience this happens only -and "randomly"- when near some human player/s. Sometimes even when dogfighting one vs one. The fps recover completely when you go far enough from the player causing the problem. I don´t think is related to graphics, but more about connection issues between the involved players.
Would be nice to know if when this happens both/all players experience the same problem or only one/some of them.
This bug is already in
http://www.il2bugtracker.com/issues/253
You should go there, add any information you have, and vote for it.