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Suzanne on transparent background with 1 sample and denoiser is rendering slower than at 100 samples and no denoiser. PC specs: Ryzen7 1700 (stock), GTX 1050ti, Win10 64bit

v.2.79a, CPU+GPU, 1980p, 64px tile:

  1. 1 sample + denoiser (radius 10) - 00:40:35
  2. 100 samples - 00:12:47

What is wrong here?

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Nothing is wrong, the denoising takes time, that's all.

In your blend, most of the area has literally nothing. The renderer notices that and renders those areas extremely fast. The denoiser still has to treat all the pixels, and does not know that there is nothing there.

As an exemple (CPU only, render size changed to 1080x1080):
100 samples, no denoise:
your file, ('as is') : ~11s (GPU 6s)
with added background plane : ~33 s (GPU: 24s)

1 sample, default denoise settings:
your file, ('as is') : ~25s (GPU: 9s)
with added backgound : ~26 s (GPU: 10s)
(render of 1 sample w/o denoise takes <1s, with or w/o background)

I'm not going to give all the results, but for this scene, the denoising seems to add ~25s to the processing on CPU (on my system, and with the reduced output size!), independent of the presence of a background plane or the number of samples.

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  • $\begingroup$ Then I guess, I am misunderstanding conception of denoiser. Is it more intended to remove fireflies where you can't increase sampling (because it is already high) rather than perform a cheaty way to keep clean render and cut some extra time by lowering sample amount? $\endgroup$ – Serge L Mar 16 '18 at 12:27
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    $\begingroup$ Well, denoising is an extra processing step, with associated costs, which are *constant *for a given situation, and it's efficient. But, to halve your noise, you'd need to use 4x the n° of samples. Your sample scene is extremely simple, so doesn't require many samples. For some scenes (interiors), I've seen sample counts of 1000s, there the denoiser is very useful. $\endgroup$ – remco Mar 16 '18 at 14:02

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