Between ACR version 2010 and 2012 (current), the software was rewritten from the ground up. It does a lot more math and processes the images with more finese...
When I used Photoshop CS5, I would watch four raw images process at once and zip right through the folder on my quad-core, even raw files from the D800.
When I use Photoshop CS6/CC the images process much slower than before but for many of them, the quality is much better. I don't get the ugly halos from the clarity slider as in older versions. Other controls when pushed to their limits used to have issues and I had to restrain myself or the image would have issues. Less so today with the current ACR.
Always seems like a trade-off, but in this case, the trade-off is for the better, IMO.