Atomic Perception

I might guess you'd have to Unpack the alembic before you can assign materials. Otherwise, you'll just be adding attributes to the points representing the packed parts, and not the actual surface.

Try dropping down an Unpack node right after the file or alembic node.

The way I understand it is that VEX wrangles running over points and primitives are multi-threaded. When you run a VEX wrangle in detail mode, it's running single core.

What first comes to mind is duplicate faces. I would critically inspect the mesh geometry.