You would require to update the Graphics driver on your machine and that could be an issue at this moment. If you update the Mac OS to the latest version, it will also update the Graphics Driver if there's pending one.
As you mention in your post, you have also installed Lightroom CC, does it doing the same which is happening with Lightroom 5.
I have the same problems with both Lightroom CC and Lightroom 5.7.
I may need to update to the latest Mac OS.
One thing that is holding me back is that I use Carbon Copy Cloner to make automatic nightly backups of the 3 computers in my studio to a server in a different building. My current server cannot be updated to the latest OS. If I want to keep my backup system working I need to update to a new server and a newer version of Carbon Copy Cloner. Updating this system is not a small task, but I will be doing it soon.
A Mac savvy friend suggested that I create a new user account to see if the problem of freezing with the second monitor would also happen with a new user account.
I created a new user account, and Lightroom works fine.
I then needed to migrate all of the settings for various programs to the new user.
I also decided to upgrade the OS from Mavericks 10.9 to El Captian, 10.11.
I have also changed from Lightroom 5.7 to Lightroom CC 2015.
All of these computer tasks took much time, but now everything is working well.
Great, thanks for sharing the solution.