Rendering issue in Develop Module

Question

Slow rendering has been discussed in many threads, but can anyone explain what is going on with this specific issue I am having?
If I am working in the Develop Module and I render two raw files at 1:1, after the initial rendering delay I can generally toggle between the two and they are rendered almost instantaneously.   I assume this is because some info has been stored in the ACR Cache and Lightroom can quickly display them.  Now if I render a 3rd raw file at 1:1 and try to toggle between 3 images, one of them always needs 10 seconds or so to load/render.  What is happening here?  I thought the purpose of the ACR Cache was to allow for fast rendering of previously rendered images.  I currently have my ACR Cache set to 10GB and I have recently purged it, so I am nowhere near the limit.  Other system/software specs as follows:
LR 5.2
NEF files from Nikon D7100 (approx 30MB)
iMac 27-inch 2560x1440
OS 10.8.5
Processor:  3.06 GHz Intel Core 2 Duo
RAM:  16GB
Video Card:  ATI Radeon HD 4670 (256MB)
Storage:  1TB SATA 7200rpm, 25% full

Answer 1

LanceBenedict wrote:
Thanks for the explanation on the RAM caching.  If I am understanding you correctly on the ACR Cache, it sounds like it is essentially useless once you have applied some editing to the image because you need to wait on those edits to be rendered. 
No, I wouldn't agree that it's useless. When you load an image into Develop you'll be presented first with a brief view of your Library preview, which should quickly be replaced with the ACR Cache "scene-referred" preview that will have any existsing edits applied before you see it. That should all happen quite quickly, and when that preview is loaded the sliders will be activated. When that happens you can start work, even though the full file is being rendered in background, and there is no need to wait until the "Loading" indicator goes out (as I said earlier, you can turn that off if you prefer).
So the benefit of the ACR Cache entry is that it allows you to start work in Develop whilst the original file is still being fully converted.
Note that although we are talking specifically about the ACR Cache entry, the same holds true for DNG Fast Load Data and/or Smart Previews. Which one of those 3 is initially used depends on what exists at the time of loading, and what file type is being used. But the same principle applies to all three.
In terms of reducing the rendering time, that's a more problematical issue as there are various factors in the pipeline which come into play, any one (or more) of which may be sub-optimal. These factors include:
1. Size of the files being loaded. It should go without saying that 36mp files from a Nikon 800 are going to take quite a bit longer that the 10mp files from say a Canon 40D.
2. Location of the files and speed of the interface, i.e. expect image files on a USB2 connected hard drive spinning at 5400rpm to be a lot slower to load than if they were on a 7200rpm drive internally connected on a 6gb/s Sata hub.
3. Once the data has been read, how fast is the CPU.....rendering a raw file will be quite a CPU-intensive task, so the faster the processor the better.
4. User expectation.....one man's "painfully slow" could be another's "quite acceptable".
With that in mind, I have no idea if what you are experiencing is "typical" or too slow. Putting it into context, all my original files are on a fast internal drive on a 3gb/s connection, I have an i7-930 quad-core processor (now over 3 years old, so not leading edge by any means), 12gb of RAM. Full loading times in Develop are now about 3 seconds per image when using 22mp files from a Canon 5D3.
Looking at the things you've tried, purging the ACR Cache and increasing its size would have no effect unless:
a) There was a problem with the existing cache, and
b) You regenerated new cache entries
Discarding old previews and rebuilding would have had no effect on Develop loading times, nor would downsizing the dimensions of the develop window.
Increasing system RAM should certainly be beneficial to other areas of Lightroom, I'm just not sure it would have a specific benefit to image loading into Develop. That might be dependant upon what else was running in the system at the time.