Having FPS lags when drawing tiles in XNA - xna-4.0

Question

I am developing my own tile-based game in xna 4.0, and i suffer big fps lags while drawing the tiles on the screen.
I've read a lot about increasing performance and i've got to this:
I store my tiles in a simple dictionary
private Dictionary<Vector2, IBlock> WorldBlocks;
private Vector2 ExactBlockPosition;
private IBlock ExistingBlock;
public void Draw(SpriteBatch SpriteBatch, Vector2 ScreenStart, Vector2 ScreenEnd)
{
// Run over all the blocks in the screen
for (this.ExactBlockPosition.X = this.GetCurrentBlockXPosition(ScreenStart.X);
this.ExactBlockPosition.X < ScreenEnd.X;
this.ExactBlockPosition.X += Global.BLOCK_WIDTH)
{
for (this.ExactBlockPosition.Y = this.GetCurrentBlockYPosition(ScreenStart.Y);
this.ExactBlockPosition.Y < ScreenEnd.Y;
this.ExactBlockPosition.Y += Global.BLOCK_HEIGHT)
{
// Check if there is a block in the exact block location
if (this.WorldBlocks.TryGetValue(this.ExactBlockPosition, out this.ExistingBlock))
{
// Draw the block on the screen
this.ExistingBlock.Draw(SpriteBatch);
}
}
}
}
private int GetCurrentBlockXPosition(float X)
{
return (int)(X - X % Global.BLOCK_WIDTH);
}
private int GetCurrentBlockYPosition(float Y)
{
return (int)(Y - Y % Global.BLOCK_WIDTH);
}
In short, this code is drawing the blocks that are on the screen only.
But still my fps falls to 31 when there is a lot of tiles on the screen.
The tile width and heigh is 20 pixels.
Any suggestions why my fps falls?