I think it has got to do something about the cores used when rendering the materials ~ The materials gives instant feedback when working on a 32 core workstation, but it's clearly visible when working on a old mac or a laptop. It's almost unusable when working on the materials.
Maybe giving some specified cores for the materials to render should fix it. Maybe it could also fix the black materials when the ipr is running
Had a similar report recently, but I was not able to reproduce it on my workstation. Not sure yet, where the problem is. When rendering material previews, there's 1 thread left out for the UI, so in theory it should not lag. Will continue investigating.
Maybe giving the user a choice to manually set the cores to render the materials will do something