Lightroom Classic and high CPU usage

Open Lightroom Classic Preferences, click the Performance tab, and look at the bottom.

Is “Generate Previews in Parallel” enabled? If it is, then Lightroom Classic will use extra CPU power to build previews in advance, especially if Lightroom Classic has been left idle.

I agree that you should wait for a few days to see if it settles down, after Lightroom Classic runs out of things it can do in advance and in the background. I only have a midrange Mac (Core i5 4-core CPU), but Lightroom Classic is currently idling at 0.3 to 40% CPU, using less than half of one of the four cores. If you have an older Mac and Lightroom Classic still uses up too much CPU at idle even after a week or so, you can try disabling some of the options that have been shown.

Around the time of Lightroom 5, the complaint was that Lightroom was both slow and obviously not using all available CPU power. Adobe fixed it so that unused CPU power is now applied to performance caching and getting things done in the background. For example, building previews in parallel means fewer blank thumbnails in the grid as you scroll, and faster response as you go from image to image in the Library module.

"I have just upgraded"

To build on dj_paige's reply: If the CPU usage is constantly above 150% even when you're doing nothing, LR may be doing things in the background triggered by the upgrade. Click on the identity plate in the upper-left corner to see which of these might be running:

You may very well have Face Detection running, which is CPU intensive and can take hours for large catalogs.

Less likely, your LR may be rebuilding previews in background (which don't always cause a progress bar to appear).  Let LR run overnight -- does it still use 150% CPU when you're not doing anything?

Address Lookup doesn't use much CPU.  And you probably haven't initiated a Sync, have you?

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Over the years, Adobe has modified the code to use more of the CPU for certain tasks. I don't think you can do anything about it, and there is no way to "fix" it because it isn't broken.