Why engine denoising raw?

In GH-1 when using jpg during long exposure, say 30s, after the 30s exposure the camera will need another 30s to denoise. But when I set just to Raw (not raw+jpg), the camera still need another 30s more after the initial 30s exposure, presumably doing the denoising process. Why the camera require another 30s since I'm taking raw. Is the noise so bad in raw that the engine require to do some smoothering? If that is the case, then it is not "true" raw.
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David I used to rely on divine appointment with the subject.
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