How improve Kdenlive performance problems?

Testing on 4:3 PAL DV.

One clip on video track 1. Added video effect "Speed", set it to 50%.
Second clip on video track 2, same procedure.
3s overleap, added a crossfade transition between the two clips.

-> Kdenlive 0.5 cannot give clean preview (when it comes to the transition, you merely see a flicker; afterwards, it's o.k.).

Using a Core 2 Duo notebook (2x 1.66GHz), 1024MB (2x 512MB), NVidia GeForce Go 7400 with "latest" NVidia binary driver, Gutsy (7.10).

What can I do?

preview quality = high
It plays perfectly on my dual-core Athlon64.
Did you enable nvidia opengl proprietary drivers?
They work perfectly.