I have noticed that with my new computer, while I am editing, my edit doesn't play back in real time at all. No matter how many times I let it run all the way through, it doesn't show me what I am working with until after it is fully rendered through encoder which is a problem for me. I am asking because I have never had this issue before on my previous laptop and desktop.
Without any exact info about your computer's specs, preview settings, contents of the project and so on this is not going anywhere. Start by updating your graphics driver and check your hardware acceleration settings.