There is likely to be two issues; the speed of your computer and the size of the components in terms of the poly count.
If you have an old low spec computer then only a new PC will fix that.
You can however work smarter with lower poly components. I rarely use full 3d landscape components these days as once you add a large number of them, even a high spec computer can slow right down. If you are using shadows and styles it will slow even more. I tend to use a combination of tree plans for plan views and 2d ‘face me’ trees for perspective views (search the 3d Warehouse for both types). You can make a component/group of all the instances of each type and toggle the visibility with scenes and/or layers depending on which view you want.
If you really must have full 3d components then use low poly examples. Again search the 3d Warehouse and you will find many examples.
A quick tip for saving the final image of a large file with styles and shadows, is to do a screen grab and paste it into an image editor rather than export the image. It will be much faster and the style will look as it does on the screen. Sometimes an exported image will change in appearance. It does help to have a large high resolution monitor however otherwise the resultant image will be low quality.