Processing speed depends a lot on what software you are using. I'm using Capture NX2 and Photoshop CS6 on a Windows 7 machine with 8 GB of RAM. It is noticeably slower handling D800 files than D300 files. File sizes are running 40MB compressed, but that file may be 120 MB performing certain operations. The step that is the slowest is rendering a new image - for example a new embedded JPEG after making changes to a NEF.
If you want to create a test, simply take an existing D700 image, resize it to the dimensions of a D800 file, and go about your normal routine. The EXIF data is not materially different - its just a lot of pixels and therefore a big file.