It is likely there are too many vertices in collision geometry

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This was a warning that was recently added to MoveIt that tells you that the meshes (*.stl, *.dae or something else) that you configured your urdf model to use (in your <geometry><mesh filename="..">...</geometry> tags) are probably (way) to detailed (have too high a face and / or vertex count).

If those are too detailed, collision checking will become slow, but the results will not necessarily be significantly better.

People typically create special collision meshes by creating convex hulls. If that is not possible (because of holes in your geometry that need to be preserved), creating a subsampled version of your meshes is a good way to do this.

The thing which worked for me was using the Decimate Modifier in Blender to reduce vertices.

I personally used the Collapse function to reduce the vertices depending upon a ratio instead of Un-Subdivide as it saturates after subsequent iterations.

References:

• Great YT Tutorial
• Decimate Modifier - Blender Tutorial