Good mornig,
many thanks to all of you. I found really usefull your advices and now it seemes there are no more problems. FPS are great and following your advices results are very good.
@presto423 This environment will be used in a computer game, so I was trying to achieve more FPS but in fact what you said is right and it risks to became an useless work at a certain point.
@Bits360 thank you for your answer because I thought directional lights were important to achieve a good scene. But considering that this environment is set at night and without moon it would be pointless I think.
@chrudimer I found your advices very usefull and you helped me a lot.
Moral of the story: thanks to all of you guys; you saved my project!