On ultra-wide monitor using 21:10 aspect ratio, the game suffers from huge FPS drops in the planes' 1p cameras, specially in heavily forested maps (Bastogne, Pogostje, ...). After some troubleshooting, I found out that the problem is caused by the ObjectTemplate.insideFOV setting. By default, its value ranges from 1.2 to 1.3, depending on the plane camera.
For ultra-wide screens, don't go over 1. The copy of the PlaneCameras.con file below changes the values for this setting to 0.99, and also bumps the gun camera (sights) FOV a bit, from 0.8 to 1. This fixes the FPS issues and aligns the FOV of both cameras.
Replace the PlaneCameras.con in <PATH_TO_FH2>/mods/fh2/objects_vehicles_server.zip\Vehicles\Air\Common.
[!CAUTION]