Trees in the scene are displayed as bounds, so viewport can be much faster. If you want to change it go to: Object properties - Viewport display - Display as - choose what type of displaying do you need

All trees and bushes are linked from other files, so the Blend file is much more responsive. However, It means they are not fully editable in the scene. If you want to adjust them follow this path in the file browser : 

your disc\Garden_01_3D Shaker\_outside_project\G\3D_Shaker\models\trees

or in Blender - select the object - press N - click Edit linked Library - click Edit Library: name of the asset

make sure that after you unpack 3 ZIP files, your folder will appear like the image below, it´s crucial.