Previews seem to be a bit of an odd thing. There's the main 3D panel which you can set to realtiem render (which is neat but naturally slow) and a few Blendertypical views (solid, material etc) as well as the material preview sphere (or other shapes) in a materials sidebar dialogue.
but they seem to work a bit odd and I'm not sure why.
1. I have to check if it's still in the actual version, but some materials would not render properly in their preview .. way to dark etc. Sometimes I get a proper result there. Also, why are octane texture based materials naturally scaled up way to far? You can have a model of 2 meters in blender, and the octane material for some "nobby rubber" has "nobs" the size of 1 meter instead of like 1 cm.
2. Is there a way to get somethign a bit more performant than realtime view that still produces a somewhat realistic view (biased/fake rendering etc)?