Yep no question LR4 is slower than LR3.6. It's agreed. But we love the functionality and the files it spits out.
Loading straight CR2 files into my LR4 could only be described as "life in the slow lane". But switch to a DNG workflow and you'll see a difference. Rather than use LR "Import as DNG" use the standalone DNG v7.1RC. This will see your 5D3 files. Be sure to go to preferences and check the fast load option. This adds less than 100Kb to the filesize. This doesn't take you straight out into the fast lane, but it's comfortably quicker. Get it here http://blogs.adobe.com/jnack/category/dng
Something I learned to avoid in LR3 was the brush tool. If I use it in any way, there is an instant slowdown, fixed only by restarting LR. If I use it heavily in LR4 the program all but freezes. I know this sounds odd from a systems viewpoint, but LR4 didn't re-stabilize until I'd done 4-5 full shutdown/restarts. No more useful brush for me!
There is a 780 post thread in the Lightroom page of the Adobe forums on this subject. I think they know about it.