If your system is having difficulty previewing I can only suggest reducing the preview quality. If you want to try different software I would check out The Canopus Edius NLE, I found it very good for the issue you are having difficulty with. There is a 30 day free trial down load from their website.
There is a way of setting the priority of a program through the Task Manager. If you right click the item you want to change and select the setting you want. I don't know if it will help though. If the system is too weak for the task it's too weak.
There are other things you can do such as having the media on a separate drive, defrag etc. these things will make the most of what you do have.
Hope this helps.