Why my game is so slow ?

Beside your configuration, who is maybe not strong enough. One of the "good" reasons why your game is slowing down during your game play and during the change of time is, the add-ons you place in your Extensions sub-folder!

This is what Wraithchild and SteveXY have to say about that:

Wraithchild: Using extensions is going to slow the game to some degree. This is most noticed when the game is loading but can also slow the game play. If it's really bad, then something might be wrong with your game.

The only way I know of to check this, would be to systematically remove all extensions then add them one at a time and start the game to see which one(s) slow it down.

SteveXY: Every extension slows the game down. Outfits in particular. Outfits slow the game about 10-times more than objects. The more you add... The slower it would be.

Also the number of objects you add is a reason. If you have many different objects in the game, it's going to slow down your game play. So it's better to use only 3 or 4 new different objects than 50! During time changes, your pc has much to do: each used objects needs a new radiosity... So it´s better to have some objects in double or triple then to have them all different.

CaféMax: I personally took out from my "Extensions" sub-folder, some outfits and some objects I was almost never using anymore, and my game play speed up. I also notice that placing my graphic options to "medium" instead of "high" and putting "off" the shadow, help me a lot too.

Maybe those tips will help you too.