Firstly, what is mapObject (Did you accidentally remove an import statement) ?
And Secondly, are all those "layers" the drawing of your houses (I cannot really tell) ?
And since I cannot run your program, there is not much I can do to test it...
But the only thing I could think of is:

By constantly changing your textures, even if you did not write the code that does it (probably mapObject does it) you have to constantly reload your textures which will eat up a lot of time.
And if you are not changing the textures of your houses, then that is why you can afford so many of them.

Essentially, try removing the part that changes the textures.
And please try to explain mapObject.

I hope this helped

Many thanks for that quick answer !

Map object is just a simple interface (I will add more methods later ) :

1. interface mapObject
   {
     void draw();
   }

And when I talk about "layers", what I mean is the shapes (i.e. 1 quad shape = 1 layer) each layer has one different texture (that represent a tree branch with leafs) and a different Z position, so it creates a beautiful 3d effect from 3 or 4 2D PNG pictures.
Houses are 3D elements drawn differently (TRIANGLE_STRIP for walls and one QUAD for the roof), using 2 differents textures (roof and walls).



More details to help you helping me :

Actually, I just create tree objects, and I add them into a mapObject array which contains all my map objects (houses and trees).

Then in the main draw() method, I iterate the array and draw all objects (I dont care wether its a tree or house).

SO I create the two trees :

1.   tree(315,2870,250,2,250);
     tree(840,3185,210,3,200);

I push them into my mapObject[] map; in a method called only once.

Then the main draw method can iterate the map array and draw the objects.


I don't understand one thing in you answer : I did show you the code that change the textures of the "layers" just look at the draw() method of my tree class (I load the textures in the class's constructor and I change the textures on every shapes directly in the draw() method ).

So, knowing that I created the two trees only once, I call only one time the constructor so I "load" only once the PImages (then they are saved in the tree's instance).

The only one thing I do on every frame is create the shapes ( and obviously applying the texture)... But the loadImage method is only called once, isn't it ?


I tried to do what you told me to do : use only one texture to draw all shapes :

1. 
         beginShape(QUADS);
         {
           texture(tex1);
           vertex(x-mySize,y-mySize,h,0,0);
           vertex(x+mySize,y-mySize,h,tex1.width,0);
           vertex(x+mySize,y+mySize,h,tex1.width,tex1.height);
           vertex(x-mySize,y+mySize,h,0,tex1.height);
           if (tex2 != null)
           {
               vertex(x-mySize,y-mySize,h-60,0,0);
               vertex(x+mySize,y-mySize,h-60,tex1.width,0);
               vertex(x+mySize,y+mySize,h-60,tex1.width,tex1.height);
               vertex(x-mySize,y+mySize,h-60,0,tex1.height);
           }
           if (tex3 != null)
           {
               vertex(x-mySize,y-mySize,h+10,0,0);
               vertex(x+mySize,y-mySize,h+10,tex1.width,0);
               vertex(x+mySize,y+mySize,h+10,tex1.width,tex1.height);
               vertex(x-mySize,y+mySize,h+10,0,tex1.height);
           }
           if (tex4 != null)
           {
               vertex(x-mySize,y-mySize,h-30,0,0);
               vertex(x+mySize,y-mySize,h-30,tex1.width,0);
               vertex(x+mySize,y+mySize,h-30,tex1.width,tex1.height);
               vertex(x-mySize,y+mySize,h-30,0,tex1.height);
           }
         }
         endShape();
   

But no changes, always 35 ~~ 40 FPS....