Slowdowns because of compounding timeline complexity (not length) is unfortunately still an issue not resolved with FCP X. See here:
http://forums.creativecow.net/thread/335/21342
I haven't seen any amount of RAM, CPU or GPU, or even HD speed helping. Resolution of the project also doesn't effect this.
To find out if it is, check the size your "CurrentVersion" file for both your event and your project. AND ALSO ANY EVENT you have in your Final Cut Events folder — even if you are not accessing media from it. I have found that if any of these approaches 80MB things slow down. Can you report back on the size?
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T. Payton
OneCreative, Albuquerque