Info here: Dreadful performance
I was getting a massive FPS drop at a specific place. While in dev mode, I watched the console, and the FPS drop happened every time just after the Seagull model loaded. This persists till you go back to the main menu as those models seem to be ‘load and stay resident’ unlike other sceney models.
As a test, I deleted it from the game files, and the FPS drop goes away.
I also got a 10+ FPS boost at my homebase of FACT.
As for the temp fix, just delete all the animals (not sure if others cause issues) by deleting everything in:
UPDATE: Bad news, latest update gets a CTD if you delete those files. The issue is still present.