Cost of weather effects and level creations

I’m new to learning ue4 and was wondering on two points renering partial effects around the player but rendering materials to show the effects, like rain make all materials look wet but the rain show up over the players camera but all materials look wet or make the rain show up in the whole area. Second is it better to have a really big map or break it apart in sections useing seamless level switching to change if making an open world map. In these two things what takes the least resources.

Thank you