If you have a ton of other programs or tabs open, close those so that your computer can focus on Lightroom. Then you can always check to see how much RAM your computer has, and how much is being used up. It may just be that your computer isn't fit to handle it.
Also, sometimes it can help to optimize the lightroom catalog if it hasnt been optomized in a while. When you open Lightroom, click file -> Optomize Catalog... and let it do it's thing.