I'm running nothing else - this computer is just for processing. I usually get through 1000 shots a day but after half a day and only 200 photos I gave up waiting for images to load and started looking for answers.
The reason why I think it's a software issue is that the DNG's work fine in 2.7 but not in 4.3. It shouldnt be a RAM issue as 12GB is more than enough (and LR only shows use of 2-3GB in Activity Monitor anyway).
And I dont think it's a drive issue as on my mac book pro using an SSD + HDD it still is slower than 2.6.
With all the added features like grain and lens correction I wonder if it's made the latest version slow?
Look, compared to drawing by hand it's pretty fast....but compared to 2.7 it's slower, much slower. When I can work & think quicker than LR can react I really notice it - those micro pauses waiting for modules to change or images to render sharp kill my workflow.
Have you tried periodically purging the previews cache? Even if you have a large enough cache I've found that it can speed things up considerably, especially with all the previews your generating every day.