SFML 2.0 - Drawing a starfield - c++

Question

I'm using sfml 2.0 to make a game. The background is going to be a starfield. Rather than use an image, I thought it might be neat to randomly generate a starfield each time the game starts up, so the background looks slightly different each time. Problem is, the way I'm currently doing it slows my game down A LOT. Does anyone know what is causing the speed hit? I've looked through the documentation to see what, but I haven't found anything...
Here's my code:
#include <SFML/Graphics.hpp>
#include "sfml helper/initialization_helpers.h"
#include "sfml helper/cursor_functions.h"
#include "sfml helper/global_event_handler.h"
#include "sfml helper/globals.h"
#include <cstdio>
int main(int argc, char* argv[])
{
try {
// some non-related preparations...
PrepareApplication(argv[0]);
float width = 640, height = 480;
sf::RenderWindow window(sf::VideoMode(width, height), "SFML Test", sf::Style::Close);
window.setFramerateLimit(60);
// Textures is a global object that has an internal std::map<string, sf::Texture>
Textures.add("ball.png", "ball");
sf::Sprite sprite;
sprite.setTexture(Textures.get("ball"));
// This is the snippet that generates the starfield
srand(time(NULL));
sf::Image starsImg;
starsImg.create(width, height, sf::Color::Black);
int numStars = rand() % 20 + 490;
for (int i = 0; i < numStars; i++) {
int x = rand() % (int)width;
int y = rand() % (int)height;
sf::Color color(255, 255, 255, rand() % 75 + 25);
starsImg.setPixel(x, y, color);
}
sf::Texture starsTexture;
starsTexture.loadFromImage(starsImg);
sf::Sprite stars;
stars.setTexture(starsTexture);
// main loop
while (window.isOpen()) {
if (Flags.isActive && Flags.inFocus) {
confineCursorToWindow(window);
} else {
freeCursor();
}
sf::Event event;
while (window.pollEvent(event)) {
if (event.type == sf::Event::KeyPressed) {
if (event.key.code == sf::Keyboard::Escape) {
Flags.isActive = (!Flags.isActive);
}
}
if (event.type == sf::Event::MouseMoved) {
sprite.setPosition(event.mouseMove.x, event.mouseMove.y);
}
// handles default events like sf::Event::WindowClosed
handleDefaultEventsForWindow(event, window);
}
window.clear();
window.draw(stars); // here's where I draw the stars
window.draw(sprite);
window.display();
}
} catch (const char* error) {
printf("%s\n", error);
exit(1);
}
return 0;
}
EDIT
I tried loading an image of a starfield and tried drawing that, the game still runs slow!
Also, by slow, I mean the sprite called "sprite" lags when following the mouse. Is that actually a speed issue?