I've just been playing with some downloaded raw files for the D800 and the 5D3 to see what all the fuss was about
Anyway, I had to download Lightroom 4.1 to be able to play with the Canon files, I never really had an issue with V4.0 in terms of performance so didn't bother with the update. But now that I have, I've found the performance improvement is huge! Even on a D800 48MB NEF file it rockets along!
So if you're a Lightroom user and don't have 4.1, go and download it right NOW! You'll love it