I have just been told that working with scenes slows sketchup down is this true ??
The only delay you might experience is when opening or saving and SketchUp updates the thumbnails for the scenes. You can turn off thumbnail generation if you want. I don’t display scene thumbnails when I work because I give the scenes names that tell me what I need to know. The benefits of using scenes, especially in conjunction with LayOut, far outweigh any possible slowdown, though.
Yes I do too - Let me look into the thumbnails now 
I have looked into outliner and using section planes but I don’t think I’ve understood the concept yet
I’m not sure what Outliner and section planes have to do with your question about scenes but both are useful in their own way. Outliner is basically a text representation of the components, groups, and section planes in your model. If you’ve got nested groups or components, it shows that and what each nest contains. It also allows you to access a component or group for editing in the same way you would access it in the model. This can be handy when you can’t easily see the group or component but you know it’s there.