MeshLab is an open-source, advanced 3D mesh processing software system.
This article will show you how to reduce the polygon/face count of a model for better performance on Sketchfab. This is especially relevant for high-resolution 3D scans/maps and heavy CAD files.
Open your model in MeshLab. For example, drag and drop the OBJ file from your 3D scan into the window or go to File → Import Mesh... and navigate to your file.
Go to Filters → Remeshing, Simplification and Reconstruction → Simplification: Quadric Edge Collapse Decimation (with texture) (if you're using a texture).
Choose a Target number of faces. Try 1 million, or 500,000 if the model quality doesn't deteriorate too much. Then click Apply.
When it finishes, Export mesh as... (e.g. OBJ) and make sure the TexCoord options are enabled if you're using a texture.
Then, you can upload the new mesh to Sketchfab.