Are you working on a Mac or Windows computer ?
Do you have any external drive connected to your system?
Is face recognition running in the background? Do you have any images stored on an external disk or on a networked disk? Check the connections on all your USB devices. You sometimes get these lockups when there are USB connections that are loose and when you have images on external disks or when your network is not 100% reliable and you have images stored on networked disks that are being accessed to generate previews. Do you have graphics card acceleration turned on (in preferences->performance)? If you do, try turning it off.
@sarsaxen @Jao vdL: Thanks for replying.
I'm using a Mac. My library is located on an external thunderbolt drive.
I do have graphic card acceleration turned on. My card is a GeForce GT750M.
Is there an issue with some video cards?
I reckoned you were using a mac from the "spinning beach ball of death" comment. Try creating a new catalog file on your internal disk and import a few images into it. Keep your external unmounted. Does that stop it from spinning to death? You can go back to your catalog on your external by selecting it from the File menu->open recent. The reason this matters is that the spinning beach ball is usually caused by disk access being blocked and that blocking the program. If your external is failing or has an intermittent connection you can get this effect.
The reason I ask about graphics card is that there were lockup issues caused by the graphics card acceleration code in Lightroom that especially reared its head on Mac OS X 10.10 and 10.11 when Lightroom was at 2015.2 and 2015.2.1. Most of those were fixed in 2015.3 but there still might be some bugs left. The graphics card acceleration only really helps if you are on a 4K or 5K display or on a retina display so you don't lose much by turning it off.