Moving directional light cause huge fps drop: why?

Moving directional light cause huge fps drop: why?

I've implemented a simple script to simulate Sun Rotation (day/night cycle). 
In my "Update" i Rotate main Directional Light (the sun). 
What i've noticed is that my FPS dropped from 60 to 20-25 fps. 
I can understand why: recalculate all light each frame can be (euphemism) cpu-consuming. 
So i switched approach using an InvokeRepeating with a repeating time of 0.25f seconds (4 "movement" per second). 
Also with this approach my frame rate drop from 60 to 50 (unacceptable). 
So my question is : is there a way to move/rotate a light (simulating a day/night cycle for instance) without meet a so big fps-dropping but mantaining realism ?
Thanks

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