insert prefab in the scene??why it becomes slow?

when I plug in the scene taken from the site of a car in SketchUp format skatchup and after I convert it. fbx, happens to me that unity becomes very slow and no longer able to play in the scene by inserting 2 or 3 prefab, instead examples of maps of unity have a lot of objects, how do I get the prefab light? or maybe you suggest a site where you can find 3d objects that do not become sluggish unity? (sorry for my english)

your prefab has too much polygons that’s why it goes slow, you have to make another one with less polygons.