What are your system specs? RAM, OS, Platform? Which version of Lr? 2019 - v8.0? How much memory do you have allocated to Lr? How much free space do you have on your drive? Is Lr installed on your internal drive in the Pictures folder?
Get that MAC to the Apple Store.
First, verify that when your LR is idle after a few minutes, it's using less than 10% CPU. (Run Activity Monitor and click on the CPU tab to see LR's CPU utilization.) If it uses more than 10% CPU when it's idle, post back here with more details.
Otherwise, your cooling vents are probably clogged with dust or your fans not working properly.
In the last several years, the number one complaint about LR (after unfixed bugs) has been that it's too slow, and Adobe has worked hard to have LR utilize as much of your processor as possible, making more effective use of the multiple cores and, for newer computers, using the GPU (graphics processing unit), which consumes a lot of power. That power creates heat, which must be dissipated.
If your fans are clogged or otherwise not working properly, or if you have a laptop without fans, the computer can overheat and then shutdown. For many people, LR is the only program they use that loads the CPU in this way and triggers a crash, so they mistakenly assume it's LR's fault.
The MacBook Air is notorious for overheating. It doesn't have fans, just a few vents. See this Google search for coping strategies: https://www.google.com/search?q=macbook+air+overheating&oq=macbook+air&aqs=chrome.0.69i59j 69i57j69i60j0l3.2119j0j1&sourc…
There are free utilities for both Mac and Windows that log the processor temperature, handy for diagnosing overheating.
Thanks for the reply. Attached screenshot is with LR minimised and left for a while (and not asking it t do anything - waited until it had finished the last process before I minimised).
Info requested by cmgap
i5 processor, 8GB ram, OS 10.14.1, 280GB free space, with LR installed on the main hard drive (not sure how much memory allocated to LR).
[View this reply in your Web browser to see the screenshots.]
If LR is truly idle, it shouldn't be using that much CPU.
But even when it is, no computer should truly overheat to the point of shutting down -- it should just run its fans to keep cool. So if the computer is shutting down, you've got a fan/vents problem. Which Mac model and year do you have?
Let's triple check that all LR background activity is paused:
1. In Preferences, uncheck the option Replace Embedded Previews With Standard Previews During idle Time.
2. Click on the identity plate in the upper-left corner:
Make sure that Sync, Address Lookup, and Face Detection are paused and no other activity is indicated in the progress bar.
3. In the left column of Library, make sure the Folders, Collections, and Publish Services panels are completely collapsed:
4. Restart LR and wait for at least five minutes.
Is CPU still way above 10%?
Brilliant thanks. Seems as though facial recognition in the background was the beast. I thought they were all done but it was still indexing.
For clarification further up, it thankfully hadn't got as far as shutting down, just very hot to the touch and the fan on for long periods. All sorted now though, so thank you.
Right, LR doesn't make it easy to see everything that it might be doing in background.