I have yet to encounter any significant performance loss (FPS) with the new version of BAT, drops to to the 30-ies, yes, a slide-show never once. And I am updating a number of old campaigns and have found out that I can double, even triple the number of aircraft in a single mission without any noticable FPS loss. In static object rich environments, yes, there can be a drop, also when there is a LOT of ground activity, flak etc, but never even close to slide-show. (Sometimes it seems to help to look skyward, and then the FPS are restored to a steady 60, but I don’t know if the effect is steady.)
Berlin has always been a FPS-killer, in ALL versions, but VP media made a new version of the city that has helped to alleviate the problem a lot.
But yes, some special environments will affect the FPS and dogfighting with a lot of aircraft over a very big city will come at a price, in all systems.