First of all, your video is AVCHD, which is the most demanding format of all on computer resources. If you look at my Profile, you will see I have a Quad core 2.6 GHz, and its preview in Project mode is also slow, and sometimes jerky, when I preview an AVCHD project. I suppose it depends on your own outlook, but I have learned to live with that since all I really want from a preview is to see that everything is working as it should. I know from hands-on experience that the end result, when I render, will be smooth. Your computer is a little slower, but you could test the final result and see. By the way, mine reacts the same way with AVCHD whether it is X3 or X4, so I am a little surprised that you though you didn't have this problem with X3 ... unless you were using some other format of video.
I found that I first ALL files must add to Create Smart Proxy File.... ? That's not user friendly. But maybe that's the thing....
The very purpose of SmartProxy is in fact to be user friendly. But you have to understand how it works. It takes a difficult format like AVCHD and creates copies of each video clip but using an easy, non-demanding standard definition format. Then you apply and preview your edits of those copy/"proxy" files, and when satisfied, the edits are applied to the original AVCHD and then you produce your final video. But that in itself is a very complex process and cannot be done in just a few seconds as you seem to think it should. So all I can suggest is that you go and have lunch or mow the lawn or something while the proxies are being created.