If you appreciate the effort, please have a look at my main weekly blog which contains my "sporadic musings and compelling images" as well as photography and post-processing tips and techniques. All designed to give you ideas! By hitting the "Newsletter" button at upper right on the blog, you'll get an email note when a new post appears, as well as free access to my eBooks and other resources.
Note: I use Lightroom 5 (version 5.7 at this writing) and Photoshop CC 2014. Some of these functions also work in earlier versions of the programs. I run on a PC, not a Mac but they're almost the same in Lightroom.
Lightroom sometimes appears to respond and process images slowly. Some of that is due to your preview settings, which you can change to match your needs and available hardware.
Right after importing new images, people sometimes complain that when Lightroom opens a RAW file, first it looks sharp, then it looks ugly for a while, then it gets sharp again, especially when viewing the image larger than grid view. What's happening?
That exact sequence really only happens on newly imported files, where Lightroom hasn't yet had time to generate the standard previews. First thing you see is the JPEG view that is embedded into the RAW file by your camera. That gives way to the unprocessed RAW file while Lightroom churns away in the background to generate the 1:1 preview. Finally you'll see the image in all its glory, including edits you've made inside Lightroom.
You may also see a delay in seeing this full quality image when you view an image in a larger view (like Loupe, or in the Develop module, or especially when you zoom in to a 100% view). Again, Lightroom needs the 1:1 preview and may ask you to wait while it's being generated.
Importing new images
In the process of importing new images, Lightroom's overall performance might appear to be slow because it's working in the background, using computer resources. One of those things is generating the full 1:1 preview you use whenever you zoom in on an image. You can improve that performance by delaying when that happens (you can do that at any time from Library → Previews in the main top menu). Here's what the upper right corner of your LR screen looks like just before you tell it to start importing:
Hard Drive Space
Now let's look at what those 1:1 preview files look like.
Previews take up some hard drive space. But not that much, in the grand scheme of things! Remember that Lightroom is just a glorified database: it does NOT contain your actual images, just pointers to tell it where to look for them, just like that card file in the old public library tells you which shelf to look for when seeking a specific book.
Now my Catalog Previews file is ONLY 13 Gb because I've chosen to manage its size. If I had full 1:1 previews from all 63,000 images it would be much bigger. Here's where you manage that.
In the same place as before (Edit → Catalog Settings), open the "File Handling" tab. You have three choices you can make about your previews.
The first line is about the SIZE of the preview.
My biggest monitor is set to 1920x1080 pixels. I chose a preview size so I could see ALL of those pixels, so just larger than the screen. If you have a lower resolution monitor, you don't need the preview to be that big.
You can also choose the preview quality. I find "Medium" to be good enough for me. Remember, you're not changing the image, just how you view it onscreen when you zoom in to edit. When I do a really fine edit, I switch to Photoshop.
Now here's the big space saver. You get to decide how long the huge 1:1 previews stay on your computer.
In my case, I decided I don't need the 1:1 previews instantly available on images I'm not currently working on. So I tell Lightroom to discard them after one week.
When you click on an image in Loupe view or Full Screen, or in the Develop or Print or Slideshow or Web module, and especially when you zoom in, Lightroom needs that 1:1 preview. So it takes some time to generate it if you have discarded it. Once it has, it'll keep it around as long as you've told it to in the above menu.
Adobe has a resource page if you want to read more on this subject. It's here: https://helpx.adobe.com/lightroom/kb/optimize-performance-lightroom.html
If you have tons of hard drive space, go ahead and keep your 1:1 previews forever. If you have limited RAM or CPU speed, you may want to hold off generating them immediately on import. Lightroom can churn away in the background if you tell it to (Library → Previews from the main top menu) or when you decide to look closely at an image. It all depends on your hardware resources and your workflow requirements.
Does that make sense?
Please take a moment to visit my weekly blog. I dare you.