A small reason for this is simply Unturned's optimization, however my map is large, heavily detailed and packed full with zombies spawning and items spawning. One thing you can do to relieve some of the frame drops is decrease your model detail (the second slider in the graphics settings).

Something I noticed with Unturned is when I look toward the sun no matter the time of day, except for night, my frames increase by 20. When I look away my frames drop by 20.