Rendering flat metal surfaces

[fixeighted]

I guess it'd be impossible to assess without knowing what sort of scene it is, so a sceenshot maybe?

[JuhaHo]

I think I cant post any screenshots, but in the scene there's one HDR environment dome light and few additional lights for reflection. The material is mostly built in Steel Blurry metal material, where I have added very slight VraynoiseTex bump map for reflection variance of planar surfaces.

[fixeighted]

Maybe highlights taking an age to resolve with the settings you have...reflections of those in other surfaces adding to that...really hard to guess.
You could maybe try reducing the power of the lighting in the hdri to see if that solves it (if that's the culprit of course).

[JuhaHo]

I have created dozens of variation and finally getting some greenish light from customer side. I would not like the idea of starting everything all over again. I was kind of hoping to get ideas on how to reduce the rendering times from 24 hours per image to few hours.