To tell you the truth, when I have the need for chroma keying I usually use Primatte in After Effects. I havent had the need to do just chroma keying in Premiere.
Although whenever I use the accelerated effects like the 3 way color correct, most of the time I get real time playback with my Quadro 4000 CUDA card.
I just seem to have a positive experience with CS6.
Tom Daigon
PrP / After Effects Editor
http://www.hdshotsandcuts.com
Mac Pro 3,1
8 core
10.7.3
Nvidia Quadro 4000
24 gigs ram
Maxx Digital / Areca 8tb. raid
Kona 3