I have a few issues with CS6. First, it seems to run slower on my computer than CS5.5 with the same project.
I have a PC with i7 quad core, 16 GB, Nvidia GTX 440 1 GB video card, 4 TB Raid 0 for video.
Second, it does not seem to shut down properly. Often after I shut it down, I cannot get it to run again without restarting the computer. Last time I looked in processes and noticed it still was running a process. I shut it down and the program would restart without restarting the entire computer.
Third, when the application is running but in the background (i.e., I am using another program such as Internet Explorer), it has this annoying habit of having the monitor view jump up and take over the entire screen. The only thing that seems to get it to go away is to hit the windows button and get Premiere Pro back up to the front.
I've noticed 6 running much better and faster than 5 for me.
On one of my two computers, I also notice that when I close PP, it gets stuck in memory. I have to kill the process manually every time. (Thank God it doesn't happen on my edit rig.)
I've had no issues with PP running in the background, and in fact I often have it so while I post here.
The GTX 440 isn't on the approved card list, so one thing to check would be that you've applied the "hack" for your card, since it wouldn't have carried over from your previous CS5.5 installation.
Other than that, yeah, CS6 is much faster than CS5 on my systems - even my crappy 4 year old HP laptop (crappy for editing, anyway).
SIDEBAR: wow, I'm so tired of having to re-login to the user forums every time I try post some help up here. I don't know if it's my browser, my computer, or something in Adobe land, but I'm too busy to try to troubleshoot it. Half the time now I click on a post in my RSS feed to respond and I'm not logged in, I just close the tab and move on. Meh. Didn't used to be an issue, and was working a couple of weeks ago (month ago?) but now I have to re-login every time.