A Neat Tip for Photoshop UsersFrom Photoshop Help Blog comes a nifty little trick to view a picture at 400% and make adjustments while simultaneously seeing how that effects the picture at 100%.
1. Open an image you want to retouch.
2. Choose Window > Arrange > New Window for...
3. A second copy of the image opens.
4. Set the magnification of one to 400% using the Zoom tool.
5. Set the magnification of the other to 100% (Double-click the Zoom tool in the toolbox).
6. Every edit you perform on the 400% version will render automatically in the 100% version.
That should speed things up.