Unfortunately I'm experiencing heavy performance issues every few moments. These always come with the Sin logo shown in the upper left corner. Does the game reload in these moments, so could my pc be too fast? Any idea how I could solve this?

(running on HP notebook core i7, Radeon HD6770M, 1024x768, default openGL, HighEnd with acc.)

Look at this page - http://www.wsgf.org/dr/sin/en

In the various issues the following one is mentioned:

Issue: The game seems to "lag" horribly and there's a SinTek logo icon in the top left corner.
Solution: You need to limit the FPS by setting cl_maxfps to a lower number than 1000 (the default) - either in console or .cfg file. 100-200 works fine for me.

It suggests that you might have such a powerful machine that it is rendering the game too quickly, so limiting how many frames it renders acts as a fix. Try and see, hopefully it will help.

Thank you all so far. Maybe I can use your tips, when I encounter the problem again. For the moment I solved it, by switching from default OpenGL, to 3DFX. First I thougt this should be impossible, because there's of course no 3DFX glide card in my notebook, but somehow it worked.

The GOG installer uses 3DFX by default, using nglide to emulate it. You can tweak the nglide settings using the nglide_configurator (should be in the game installation folder, but there may be a link to it from the GOG shortcuts). With it you can push the resolution up even higher, and it has a vsync option which is probably what solved this issue for you as that would prevent rendering too many frames.