If normal FPS is 30 and you're flying a helicopter, tilting the view down / up from horizon level degrades FPS linearly with the relative angle. -30 degrees will degrade the experienced FPS below 1 and I have not dared attempting -90 (vertically) when this occurs yet.

Tilting the view -45 resulted in a 2 minute freeze of the game without any helicopter movement and at most 12 updated frames rendered (0.1 FPS).

I experienced this pre-1.48 too. This does not occur every time you fly, it seemingly occurs at random. In the same instance the issue can be absent in one moment and extreme on the next attempt.