I just updated from LR4. Not noticing any substantial changes in speed, but I am operating on an outdated PC I built in 2008 (with a few upgrades since).

I have been playing with the facial recognition. Seems to work ok. Although after I tagged the first person (my wife) it pretty much thought every other person was her. And it also seems to get tricked by patterns sometimes and identify random objects as people. After you put in the initial work to identity everyone, it does give you a nice way to filter images.

Haven't tried the HDR or pano stitching yet. Curious to see if the HDR is any better than the Merge to 32-bit HDR plug in that I use now.