Ok, this was not it. You indeed should try to change the "enable background draw thread" in the preferences too, it might change some things. To push the sliders to the maximum probably will not do the trick because this is not something that influences the material previews, as far as I know.

The "Render materials using Poser shader tree" indeed - when I remember it right (I never use that) - gives only icons as material previews and does not show the actual material. To see whether this is part of the problem, you can try to import another format - obj or 3DS for example. And see whether you have the same phenomena.

One day your ship comes in - but you're at the airport.