I found a solution to this problem for me.
I’m rendering with HDRi plugged into the world node. It seems like the thumbnail renderer uses the hdri too for its rendering and I’d assume its ALWAYS loading up the hdri when rendering each individual thumbnail. When I unplug the hdr the thumbnails get loaded very quickly again.
So - this worked for me. I’d assume that this is a bug in how Blender manages the thumbnail rendering - not ignoring the hdri as it is doing with the other preview.
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