Following on from @set, perhaps your preview setting of Good (half) is to high. Reducing that setting to Preview (auto) or Draft (auto) may increase performance.
You really need to explain more about what you're doing. What resolution, what computer you're on, etc.
However, my guess is the same as others, your computer is too slow to show the video preview at the quality you've set. Try setting the Preview Window to Draft/Full and see if it makes a difference.
Another check is to just render out the area where you're seeing the problem and see if it renders correctly. If it does it's almost certainly slow preview.
Is it only the pan crop changes that lag behind? Try building proxy file for that clip, and then set the preview to preview full and see if it still lags like that.