title: Tile engine performance issue
body: <p>I am using SFML and I can't find any explanation on how the library is working and I have performance issues.
  I am trying to make a tile map made of 50x50 tiles that are each 20x20 pixels wide.
  1) Each tile is a sf::sprite whose graphic image is simply a big image (my tileset), on which I set a 20x20 subrectangle.
  2) I am simply looping over total number of tiles and using App.Draw(Map[i][j]); to draw each tile, and I am using "view" objects so I can move the view around</p>
  
  <p>Now I have a very low FPS (1 image / second) and I am wondering 2 things in relation to the previous points.
  1) Is it that each sf::sprite takes time to draw because their image is coming from a huge image that was cropped?
  2) Am I right to loop over the whole set of tiles, even the ones I am not seeing? I am assuming that the view object makes it so that the tiles that are out of view are not re-computed in vain</p>
  
  <p>thanks</p>