JFrame starting up slow

Hello there,
I have a JFrame set up as a display for my Java2D game. It all works perfectly except for one thing: If I add the following code to my render loop the frame doesn’t render for about a second.

	private void drawFPS(Graphics2D graphics, int fps) {
		if (hideFPS)
			return;
		// graphics.setFont(font);
		graphics.setColor(Color.WHITE);
		graphics.drawString(fps + "", 5, 15);
	}

So without using this method in my rendering loop the window starts up and the rendering start immediately. But using this method in the loop makes the frame show it’s standard background color (this litle dirty white) for about a second before the rendering results of the game loop get displayed.
What can I do here? I have had this problem in other occasions but until now, hadn’t figured out what exactly the problem is.