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If you have modeled a highly detailed object in ZBrush, you should consider retopologizing and extracting a displacement map.

Memory usage and performance

Polygon meshes

• Shortest time to first pixel (TTFP) if not displaced.
• When displaced, the mesh will be diced and micropolygons generated.
• Un-displaced heavy meshes use the same amount of RAM, whether they are full screen or 2 pixels high.

Subdivision surfaces

• Slightly longer TTFP but dicing is multi-threaded and efficient.
• Use a bit more memory than meshes for the same number of vertices.
• A sparsely modeled subdiv will only contain the base mesh faces when 2 pixels high.
  • This adaptivity lowers memory usage and speed up raytracing.