Windows Photo Viewer is insanely SLOW!
SOLUTION - At some stage you probably color calibrated your display using the tool in Windows 7. As soon as you remove this calibration profile speed will return to normal.
Right Click Desktop -> Screen Resolution -> Advanced Settings -> Color Management -> Color Management -> Advanced -> Change System Defaults
Then click on the default profile (the one you are using now) and click Remove. The profile should have some string in it like 'calibrated' ( I forget, I just deleted mine ). Make sure the new default profile does not have the 'calibrated' string in it. When you click Remove you will probably note some change in the screen colors as another profile becomes the default.
Now suddenly the image viewer will be lightning fast.