I am not sure if this is campaign specific, but I don't get this problem with any other missions or modules (although they are usually shorter).
It started with mission 7, all is good untill more or less passing Irish and heading towards B4 and M5, FPS gradually drops to finally reach amazing 1 FPS which is complately unplayable.
I am not sure if there was an update to DCS which affected how the missions work?
I guess it is connected to other uints doing their job, engaging air defences etc. which makes everything to slow down so much.
I've tried it in both OB and release, with various settings, with or without TrackIR, tried quite a few things. CPU, GPU and memory are not overloaded either.
Currently I am not able to finsh the campaign because of that :-(
Any tips would be appriciated!