Drawing lines is incredibly slow?

[Sub]

I'm currently trying to replicate this tutorial [http://gamedev.tutsplus.com/tutorials/implementation/simulate-fabric-and-ragdolls-with-simple-verlet-integration/] and I'm getting low FPS when I try to draw a grid. 

I have 2500 points on screen, each spaced 6 pixels apart from each other, and I want to draw a line between each point and its neighbor.  When I do so, my FPS goes from 4200 to 20-100.  Is this normal, or am I doing something wrong?

Here's the code I'm using to draw the lines.

void SFML_Program::drawLine(sf::Vector2f a, sf::Vector2f b)
{
        sf::Vertex line[2] = {a, b};
        MainWindow.draw(line, 2, sf::Lines);
}

I've also tried:

        void SFML_Program::drawLine(sf::Vector2f a, sf::Vector2f b)
{
        sf::VertexArray lines(sf::Lines, 2);
        lines[0].position = a;
        lines[1].position = b;
       
        MainWindow.draw(lines);
}

I'm calling that 4900 times to produce this. 

http://i.imgur.com/tI1tz.png

The code that calls it simply retrieves the two points it needs and then calls the drawline.  Am I doing something wrong?

[Laurent]

Drawing 4900 lines this way requires 4900 OpenGL calls, to render 4900 tiny geometries. To get good performances, you'd rather draw one big vertex array containing the 4900 lines, in a single call.