Without looking at the document it's not really possible to say with any certainty what is causing the performance problems you see. It could be a chart expression, but it could be a data model problem as well. This blog post might help troubleshoot performance problems in your charts: Recipe for a Memory Statistics analysis
On a sheet objects can be assigned to different layers so that objects can be stacked on top of each other in the order you want them to. You can set the layer setting of an object on the Layout tab of the objects properties window.