I have a very large UV map for a house of mine. Due to the nature of the shingles being single duplicated objects, I have quite a giant UV map when exporting and importing for Unreal Engine 4. Does the giant UV map have an effect on performance at all when playing? Or does it only matter when the lighting is being built?
A UV map is a set of coordinates representing the vertices of a mesh in 2D space. It is not usual to talk about it in terms of 'large' or 'small' as its size/density mostly depends on the size/density of the mesh. You should generally not care about resources used by UVMaps. The size of textures (like lightmaps) or the resolution of the whole mesh are usually the things you should worry about.