Nothing obvious stands out. Have you rebooted?
Are the software you run other than Premiere also running more slowly? From within PPro, are encode times worse, if its possible to compare?
Were you possibly overclocked and maybe you lost your overclock due to some change in the BIOS?
Rebooted, no change. No noticeable chugging on other applications (including Crytsis 2). I've never overclocked, so that's not an issue.
It did occur to me that, maybe, one or two of the video streams may have acquired an unwanted video effect: I've found that I can accidentally change tyhe opacity if I click on the timeline in the wrong way. But everything seems OK.
But I truly wish that my system could handle hi-def video well. It's a fast system. Heck, I should be able to handle multicamera editing without much of a problem. Only ways to make it faster are a new CPU or maybe a RAID drive system. But this problem was very, very frustrating.
It's a fast system.
Comparatively speaking, it's actually pretty slow.