Maze Drawing (better performance) libgdx

Question

In my game, I'm using the ShapeRenderer class to draw a maze. Basically, I'm using the rectangle function (in the ShapeRenderer class) to draw small black lines. In the past, I had no problem debugging the game performance-wise (fps = 60). But lately, I've been having some performance issues. To make it short, I took out every sprites and actors I've drawn in the game and decided to draw the maze ONLY. Everytime I debug my game through the Desktop Launcher, the fps performance lowers by half (basically around 33, 34 fps). Yet, when i run it, it goes up to 60 fps.

I believe that it's a clear indication that the ShapeRenderer class wouldn't be the best choice for me to draw the maze performance-wise. I've tried with a spritebatch with texture png image (rectangle) and that didn't change a thing. I was wondering if there was a better choice to draw the maze and still maintain an fps performance to 60 (in debug mode) or is it just normal that debugging the game would lower my fps performance by half?

P.S: This is my code which is inside the render method to draw the maze:

for(int x = 0; x < rows; x++){
     for(int y = 0; y < columns; y++){

         if(this.grid[x][y].north.hasWall){ //NORTH BORDER LINE
             shapeRenderer.rect(22+((GENERIC_WIDTH_HEIGHT_MAZE+10)*x), 450-((GENERIC_WIDTH_HEIGHT_MAZE+10)*y), GENERIC_WIDTH_HEIGHT_MAZE+10, 0, color1, color2, color3, color4); 
         }
         if(this.grid[x][y].west.hasWall){ //WEST BORDER LINE
              shapeRenderer.rect(22+((GENERIC_WIDTH_HEIGHT_MAZE+10)*x), 450-((GENERIC_WIDTH_HEIGHT_MAZE+10)*y), 0, -GENERIC_WIDTH_HEIGHT_MAZE-10, color1, color2, color3, color4);
         }
         if(this.grid[x][y].east.hasWall){ //EAST BORDER LINE
                  shapeRenderer.rect(22+((GENERIC_WIDTH_HEIGHT_MAZE+10)*(x+1)), 450-((GENERIC_WIDTH_HEIGHT_MAZE+10)*y), 0, -GENERIC_WIDTH_HEIGHT_MAZE-10, color1, color2, color3, color4);

         }
         if(this.grid[x][y].south.hasWall){ //SOUTH BORDER LINE
              shapeRenderer.rect(22+((GENERIC_WIDTH_HEIGHT_MAZE+10)*x), 450-((GENERIC_WIDTH_HEIGHT_MAZE+10)*(y+1)), GENERIC_WIDTH_HEIGHT_MAZE+10, 0, color1, color2, color3, color4);
         }
     }
 }

Any insights would be appreciated. are the following values:

GENERIC_WIDTH_HEIGHT_MAZE = 26 (Integer)
rows = 9
columns = 12
color1 = color2 = color3 = color4 = Color.BLACK

Answer 1

Well, in the end, I made a TextureAtlas containing the walls drawn (small png pics) and called two of them (one horizontal and on vertical) to draw the maze and got rid of ShapeRenderer. Like Deniz Yılmaz mentionned, ShapeRendereris only used for debugging which is probably why the performance slows down inside the for loop. I also a couple of other performance optimization on other parts of my code. Now the performance is at 60 fps all the time. Thanks everyone!