When this is happening you might want to enable developer mode, open the fps window, and watch the VRam allocation. I’ve noticed the same kind of sudden drops over the London area and every time FS2020 was oversubscribing the available VRam. When it does, it must discard allocated buffer to stream new ones and it does this every frame which is certain to make the fps falling down. Once it recovers fps rise up again to previous levels.
I don’t know if there is a new bug in their VRam allocation manager, or if this is related to bugs in the mesh causing their mesh optimizer tripping and sending tons of unnecessary vertices trying to fill holes (I’ve documented one hole in the London mesh area in these forums), or something else, but this oversubscription is new to me in FS2020 since WU3 and is happening only over London area so far, not anywhere else in UK where I’ve flown. (@Jummivana sorry to bother you but this might be something Asobo devs might want to look at as well [World Update 3] London Photogrammetry Mesh Holes )