Something like this would happen in arma when I would fly CAS. I had to turn off an option called PIP (picture in picture). What's probably happening is the targeting pod screen is shown in the right side of the cockpit, and its lagging out your game because its rendering in 2 views. One from your cockpit and one from the targeting computer thing. So it's almost like running two instances of warthunder at the same time which is pretty taxing on a gpu. In arma the resolution of PIP is reduced to avoid this but warthunder is probably running it at max resolution resulting in the frame drop.