Oleg wrote:Render additional passes takes too much time. More precisely, I can not even wait for the calculation of one frame. What could be the problem? Individually the beauty pass and additional passes are displayed normally

I had this, and my problem was I was setting Max. samples in the Kernels tab of Octane Settings to a really low number, like 8. Increase this, and maybe even make sure Info passes Max. samples is also a bit higher.
Oleg wrote:Maybe this issue is known also — why picture in viewport and in picture viewer is a bit different? In PV it's more kinda contrasty (I work in linear CS)
Have you checked Render Settings -> Octane Renderer -> Overwrite kernel settings tab to make sure it's not enabled? Other than that, it could also be Image Color Profile and Tonemap type under Render Settings -> Octane Renderer -> Render passes tab.