Noticed in https://www.facebook.com/groups/CapturingRealityArena :
“For nicer rendering in Sketchfab: export high res mesh, simplify and texture, load high and low res mesh into XNormal and create normal, ambient occlusion and/or cavity maps. I usually upload to Sketchfab while XNormal is doing its thing. Tweak the 3D settings in Sketchfab and then load the extra maps when they’re ready.”
Google XNormal - “is a free tool to bake texture maps ( like normal maps and ambient occlusion )”
Don’t know if this helps - way beyond me!