What is the 'performance settings' in Lightroom CC and how can I best set for 'Camera RAW' images?

I think there was a theory once that a larger cache size would help, but more recently that theory was retracted. Now the conventional wisdom about the cache is along the lines of what Jim said, and is backed up by an article at The Lightroom Queen, which might provide some more insight into the performance settings overall: Lightroom Performance – Previews & Caches

Ken, there is no longer any program called "Lightroom CC", there is only the cloud-based Lightroom and the local system-based Lightroom Classic. You have posted your question in the Lightroom forum, but I suspect you are really talking about Lightroom Classic (both Lightrooms have a Performance Tab in their Preferences, but only Classic has a size option for the Camera Raw Cache).

So, assuming it is Classic, then there would be little point in changing the size of the Cache to 200GB. That cache is where Classic stores a special preview of each raw file that you import, and it's that preview which is initially loaded when you open a raw file in the Develop module, and thus let's you get started on the editing while the actual raw file is converted in the background to create the full-size preview which replaces the initial cache entry. The size of each cache entry is relatively small (average maybe less than 1 MB each....I've got 17,000 entries in my 10GB cache), and the overall size of the Camera Raw Cache that you set will determine how many individual cache files it can store. The important thing to understand that the cache operates on the basis of always adding the new images to the cache, and only dropping out the oldest entries if space is needed for the latest entries. So, 5GB would typically house maybe 5000 entries or more, but they would be the last 5000 that you imported. There may be a case, depending on your shooting rate, to increase the size of the cache to 10 or 20GB, but I wouldn't think it need be set any higher than that.

Having said that, setting it really high isn't an issue if the drive has adequate free space, though the space isn't allocated immediately to the cache, it will grow as needed up to the maximum size that you set.

But to be clear, the Camera Raw cache only effects the initial loading of raw images into Develop, so if you're looking for a performance boost from setting the cache to a larger amount you probably will never notice the difference.