(sorry my english is not very)
I tested FPS in a deep forest. I stood in one position without moving the camera and not turning ON any: (flashlight, chemlight, torch, etc)=> i know they eat a lot of fps in the night forest.
70 fps- at daytime (all day)
58 fps- on sunset
45 fps- at night (all night)
58 fps- at sunrise
70 fps- at daytime
And yes, a strange thing: when I turn ON Night Vision Goggles, my FPS returns to a playable 65 fps. I also noticed (if i am right) that when you turn ON the NVG, your "personal night light" automatically turns OFF. (And yes, want to say, the FPS drops at night everywhere, not only in the forest.)
Probably FPS drops caused by "personal night light"(not sure) which turns ON in the evening, and if so, then please make a function to turn OFF this stupid thing "personal night light" on client side.
Your night time light source are broken, and eat extremely many FPS, especially in the forest, especially when you turn ON any of: flashlight, chemlight, torch, etc. near the trees.
(played without mods on official servers)