Hi,
no, it's not a performance issue. For the experiment at hand I simply need mirror-images from letters. Now I could prepare these letters with some graphics software, save them as images to file and load them with sfml. This requires some work especially when I decide to use a different set of letters, or digits or want to change the image size without loss of quality or want to use a different font. Thus, it is easier to render the text with sfml. Since FlipX and FlipY are not available for sfml strings I need a workaround to create the mirror images. With the help of CopyScreen (and having in mind the coordinate issue) this workaround is available now. Fine.
Thanks - Sven