AMD athlon 64x2 4400+, 2GB ram, 9800GT. What are your system specs?
Total events in the game is over 7000, not exactly sure how many, and construct recompiles all of them each time. The 3500 are in one event sheet, not including includes, which put it over 4500. As I mentioned, if I delete all of the code then preview it previews in less than a second. It's definitely the events affecting the preview time.
The resolution of sprites might affect the layout editor, but it doesn't affect the event sheet editor. I don't work in the layout editor much, so I'm not sure what causes speed decreases there.