What exact version of Lightroom? What version of Windows? How much RAM does your computer have? Is this a new phenomenon, or have you always have this problem with Lightroom? The more details you can provide about your computer system and what is going on, the more likely it is that you will get some meaningful suggestions.
I asked for the exact version of Lightroom. I have to assume, since you didn't indicate, that you are using Lightroom 4.4. I have to assume that you are using Windows 10 because you didn't indicate. Please indicate how much free disk space you have on your hard drive. Please indicate if there are any error messages. The only suggestion I can make at this point is, in the preferences, performance section, if the graphics processor is enabled then try disabling that option and see if that makes a difference.