Comment by Const-me

> What can it possibly be doing?

Possibly, two things.

1. Indexing the mesh. STL files don't contain meshes, they instead have a triangle soup. Indexed meshes are more efficient for rendering, they save VRAM bandwidth and vertex shaders.

2. Computing normals. STL files have per-triangle normals (can be complete garbage because most software ignores them), for smooth surfaces you want per-vertex normals. Computing them well (like I did there https://github.com/Const-me/Vrmac#3d-gpu-abstraction-layer ) is slow and complicated.