Hi, the warning is from convex mesh creation in PhysX engine. You could ask about mesh collider in unity physics forum.
I have also found this thread which seems to be about the same issue.
Potential workarounds that I can think of include
- import the mesh at a smaller scale factor, and scale up the game object instead. This might allow lower resolution mesh collider with less polygons to be created
- cleaning up / simplifying the mesh in modelling software and reimport
- provide a custom mesh collider