Everything you download from the warehouse should be quarantined, and examined.
I bring it into a new model, and check for layers and materials. Everything in that component, (with rare exceptions, should be on Layer ).
There is no reason to have, for example, a car’s tire, wheels, dashboard, glass, seats, and body panels on different layers. if you need one of those cars, because it is going to appear very close to the camera in a specific scene, clean it up. Get everything on layer zero, look for over-abundant textures…try to make all body panels the same material ., renamed as “VEHICLE-PAINT-001” for instance.
Then later, you can edit the color globally. Better yet, make anything that needs to be color panels, default material, and everything else black, glass, whatever. Then you just “paint” each instance of the car with different material/colors. Sometimes there are so many embedded materials within materials that-it’s amazing SketchUp DOESN’T crash! Or overly detailed tires or wheels. Simplify all that stuff if you are going to use it repetitively, and then save it, cleaned up, in a special archives wherever you save stuff.
And if you know that cars, or trees, or books for example, are not going to be seen very closely in the majority of your views, then for gods sake don’t use highly detailed components in those areas. You’ll slow down your model for no added benefit. There are plenty of low-poly components for trees and cars that will do just fine when seen from a distance. Also, as other guys have said, bookshelves…use a component that has an image of books of the front face…maybe 48 inches long or so… there are some good ones on the warehouse. Sometimes I will take on e of those, and, in 3D, vary the height of the different books so it looks more realistic…but still, it’s mainly all one rectilinear solid…just the jpeg image on the front makes it appear as if it’s a stack of many books…and even close up, this will suffice.