I ran into a slightly strange performance bug today, not so sure if it's a bug or a consequence of my scene... basically I'm uploading a scene for myself that I wanted to view in VR, just a little bit of forest. Modelled all in Blender, and quickly added some leaves to the trees with the hair particles. Really basic low poly ones, only made of 7 polys each. To make sure they showed up in Sketchfab, I converted them into individual meshes. When I uploaded that scene, there were probably close to 1000 of these individual leaf objects, and it caused a ton of lag. It was running at what felt like 5-10 fps. I found it a little strange, since the scene is only 67.2k faces, and I have high poly hardsurface models on Sketchfab with 300k+ faces which run perfectly smoothly. So I went back into Blender, selected all those individual objects, and joined them into a single "leaves" object. So, same number of mesh islands, but all joined into one object. I just uploaded that scene, and it runs buttery smoothly.
No change in the number of polygons. Only a change from individual objects to single objects. Perhaps that's something to consider for Sketchfab performance? The render engine doesn't seem to handle large numbers of objects nicely, even with the exact same polygon density.