I have a question about CPU resource usage with LRC

I won’t claim that this reply explains everything because there might still be a bug or issue somewhere, but…

The two applications use resources differently because of two main things having to do with the fact that they are fundamentally different types of image editors:

Photoshop often only has to worry about one image at a time, at maximum it only has to worry about the number of documents open which for most people is just a few. Lightroom Classic has been increasingly optimized in recent years to respond to complaints about delays going from image to image, so it can now do some things in the background to pre-cache and pre-render nearby images in the filmstrip so that they are ready for display sooner if you go to them. (This is part of what preview rendering is about.) And of course there can be many more images in a Lightroom Classic filmstrip than most people keep open in Photoshop at one time. So while Photoshop might be done setting up and caching what it thinks is needed for the open documents, Lightroom Classic might be looking around and seeing what it can work on using CPU cores and RAM that might otherwise be idle and wasted.

Photoshop (mostly) works with straight pixels. You open an image, the image is usually flat RGB pixels. Photoshop doesn’t have to do much additional thinking about re-rendering through extended processing pipelines unless you’re using many adjustment layers or smart objects. However, Lightroom Classic must always render non-destructive edits to camera raw images from raw data that is not yet even RGB, and every time you make an edit, all affected areas must be run through that entire pipeline again to make sure the results are up to date. So compared to Photoshop, Lightroom Classic (or Camera Raw or any other raw processor) always has at least one more rendering stage to process every time you make an edit. This naturally means raw processors have higher resource requirements to maintain responsive performance. And again, they often try to use otherwise idle hardware resources to pre-render and allocate buffers and caches in memory or in storage so that you don’t have to wait as much. (This is partly the function of the Camera Raw Cache that Lightroom Classic also uses.)

You do have some control over this in Lightroom Classic. If you look through the settings, there are numerous options that let you control what Lightroom Classic is allowed to process in the background, including but probably not limited to:

Status in the identity plate:

• Assisted culling
• Address lookup
• Face detection

Preferences, General tab:

• Replace Embedded Previews with Standard Previews at Idle Time

Preferences, Performance tab:

• Generate Previews in Parallel

Catalog Settings, Metadata:

• Automatically Write Changes into Sidecar Files (this should usually be left disabled)
• Automatically Detect Faces in All Photos (same as its pause icon in the Identity Plate status area)
• Analyze Photos of Selected Source When Assisted Culling Panel is Open
• Automatically Analyze All Photos in Catalog (same as its pause icon in the Identity Plate status area)

I leave some of these on, because if a CPU has 18 CPU cores and editing the current image only needs a few of those, I don’t mind if Lightroom Classic uses some idle CPU cores to get going in the background on pre-rendering its performance caches.

Also, CPU and memory usage are not the only important components to watch. For example, low CPU usage might be expected if a process is mostly GPU-intensive. If you only watched the CPU you could reach the wrong conclusion that the application is not busy, but you would see more of the whole picture if you also looked at GPU usage and it turns out to be very busy. This was less true in the past, but both Lightroom Classic and Photoshop are now taking more advantage of the GPU. But they employ different combinations of CPU and GPU usage (partly because they’re fundamentally different types of applications, as explained), so not directly comparable.

On top of that, as we get more AI features, it can be more challenging to see where an application is busy. For example, AI features tend to be GPU-only, so how busy they are might not show up in CPU usage at all. If an AI feature can use specialized components like the neural engines in some newer processors, then watching the CPU and GPU might miss the processing going on in the neural engine. And if the AI feature is processed in the cloud, there might not be much activity visible on the local hardware at all.

So you’d want to take all of that into account.

But why on earth is the CPU jumping to 30% when I simply click on an image in the film strip?

In the Develop module, it’s probably looking to see if the image you clicked is already in the Camera Raw Cache, if it is, it’s busy pulling it out of there. If it isn’t, it’s probably rendering all currently applied edits from raw through the pipeline to make sure the Develop preview is visually complete and up to date.

In any other module, it’s either busy loading the preview from the previews cache, or if there is no preview, it’s busy building one to make sure the preview is visually complete and up to date. This is not the same preview (and not as accurate as) the one generated for the Develop module and stored in the Camera Raw Cache.

Again, Photoshop has an easier job, it just reads the rendered RGB pixels and slaps them on the screen in one go. If the Photoshop document doesn’t contain a composite (Maximum Compatibility) preview, then you might see a little more CPU/GPU use as Photoshop renders the layers to an up-to-date composite for you to see, but again this is less work than rendering from raw data.