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.“