I tested a little and I think that other light sources affect the low FPS. During the day when you can't see that there is a light on somewhere (reflectors of cars, reflectors on bases or kerosene lamps) but it still affects the fps ( three base reflectors decrease fps form 60 to 45 during day ) Sometimes randomly during outside cities and bases, maybe afected by the position of the sun. Anyway, before the last update it ran much better and never dropped to thirty. Tested on an empty official server