TIP: Open a flattened version

If you have a "huge" layered file that takes a while to open, you always have the option of opening a flattened version of the file: use File>Open, click once on the file name and then hold down Option and Shift (PC: Alt and Shift) when you click Open. A dialog will ask if you want to "read the composite data instead" - Click OK.
Just make sure that if you make any changes to the document, use Save As to save a copy - otherwise you'll save over your layered version.