With respect to your model size of 7 meg or so, be advised that the file size saved on disk is not a true index of your model complexitiy when you have multiple component instances. When you are actually working in sketchup all those trees are in memory -- a better index of true file size can be found under model info/statistics (with show nested components checked) -- namely the number of edges and faces. Some time back Alan Fraser posted a famous example of a model with 100 corinthian column components -- only 100 K or so "file size" but would bring the most powerful computer to a crawl or freeze.
That said, the advice to put your trees on a layer and hide that layer when working is sound, along with keeping shadows and textures turned off until you are ready to render.
