Do you have GPU acceleration enabled? Are you using accelerated effects? Hardware-accelerated Mercury Playback Engine only accelerates certain things--using non-accelerated effects will pretty quickly slow down playback.
Yes, I activated the GPU acceleration... and I'm using the premiere effects (levels, color correction, etc).
Thanks for the link Todd.
It seems the whole marketing around the engine, cuda, nvidia, etc got me into thinking investing money on a graphic card would make premiere faster, a lot faster. When in fact it only makes certain things faster... and on my current projects this is not even noticeable.
At least After effects previews got faster...
I should have kept my old card and invest that money somewhere else.