Thanks for your suggestions, guys. I just made a test between my old implementation via SDL_gfx (SDL 1.2) and the above and - heck - the new implementation is even faster. SDL 1.2 needs about 1200 ms for 500 filled and outlined circles. SDL2 (and my code) needs for the same task about 650 ms. So I think I can live for now with the simplest implementation. And now the test without the "normal" circles; SDL2 wins again. Maybe that's because SDL2 is faster or maybe because SDL_gfx's circle algorithm is slow.
BTW, the screen was redrawn after each circle.
Oh, I should have tested after the complete task was redrawn ... SDL1.2 -> 6(!) ms, SDL2 ->500ms
Ok, I took the approach of Super Vegeta and it's only about 18 ms now. Thanks.
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