What is more performant and has less draw calls, providing that you have less materials than UV maps overall?
In general, a material per mesh is its own draw call. There are also cases where engines may split a mesh because it goes past a vertex/influence limit. For mobile games it is generally recommended to create texture atlas’ and map all your environment assets to them. For final optimization, batching environment assets together to a unified mesh would ensure one draw call for a huge area. Some engines do this batching for you, but sometimes it is better to do it manually.
If you have a mesh with 3 materials, it would be 3 draw calls. Imagine 100 characters with 1 or 3 material slots. That would result in 100 or 300 draw calls.