Julie Kmoch from the Lightroom Team has weighed in on a discussion on the Adobe Forums relating to slowness in Lightroom Develop:
“Wait – let me explain what we found first as this may help others.
The pattern you need to be most careful of in using Lightroom: lots and lots of brushing or spot heals are what typically cause problems. Remember that Lightroom has to re-apply every edit you make every time it renders a photo. And we render with every slider change, every crop, etc. The more local corrections you have, the ‘heavier’ the photo gets. This is different from how Photoshop works, where edits typically get baked right into the file as you work.
If you look at your History panel for a photo and see hundreds of steps, you’ll get some quick relief by clearing the history panel. Your latest settings are still saved, you just can’t jump back to a specific point in time once cleared. Alternatively, if there are specific points you want to save, you can always make Snapshots for those stages and then clear the history.
If you’re having to do serious masking work on a particular shoot, such as replacing backgrounds, Photoshop might be the better tool if you have access to it.“
I’m finding in LR 5.5 that even if my history is not that large (that is, fewer than 20 items, sometimes as few as 10), LR is VERY slow to update the image (3-10 sec delay). I see a real spike in CPU usage (I have a menu item that gives me real time CPU usage). I’m running OS 10.9.4 on a 2010 Mac Pro with 48GB RAM, so this shouldn’t be a “lack of horsepower” issue.
Memory and CPU wont make that much difference as the Raw generally has compression, meaning the whole file has to be read in order to render-something that needs to be done on every change you make. The demosasiced file that sits in the Camera Raw Cache after Preview generation will help speed this a little. If you want full use of all cores, then DNG is the way to go. Current DNG (vs camera DNG) are tiled and have fast load data, so read in faster and can have many cores rendering the file at the same time. Larger files like that of the D800 also simply take time to process. Use any Raw Processor and render a file to TIff/Jpeg and see how long it takes. That’s what Lightroom has to do each time. Compare a new file to a file with Lens Corrections and Spotting. I’m sure you’ll notice the difference. Those tasks simply take a lot of CPU.
This is so very true because I just noticed how much slower LR5 was immediately after I had just heavily edited a photo with the brush, spot, heal and the other filters along with creating virtual copies, round tripping through external editors and it was just so obvious. So, I started deleting the huge history file and it was much faster moving between those photos because it didn’t have to load all of that history that I accumulated and so much of the history consisted of me applying an edit and then undoing it so many times again and again.
Jeffrey Friedl had a great tip on deleting History on images to reduce the size of the catalog too. http://regex.info/blog/2014-06-26/2434