Prefab 3D models often lag during manual retopology due to high original polygon counts or unoptimized geometry (like non-manifold edges or hidden faces).
High poly counts mean more data for your software to process in real-time, slowing down brush or edge tools. Prefabs—especially from asset packs—may also have leftover merged meshes, duplicate vertices, or active modifiers that add unnecessary overhead.
Try simplifying the prefab first with a decimation tool (preserving key details) or deleting hidden geometry; this reduces the load and makes retopology smoother.