Hi,
As far as #1, my guess is that the GPU is unable to display the image at full size. If you try a JPEG, does it work any better?
For #2, the Preview Size option controls the size that previews are made after you edit an image. If the image is not edited, then the app will use either the embedded JPEG or the RAW, depending. The app will sometimes zoom to 50% rather than 100% when you are not in Edit, but once you are in Edit, it should always go to 100%.