Project Monitor Slows Down through transitions

Hi there.

I'm still fairly new to editing but as I try to play back my project, the project monitor becomes very slow through transitions - Particularly composite.
The behaviour is identical to http://www.youtube.com/watch?v=jemvS942IJY [2], (though that's not my video).

I'm running...
kdenlive 0.9.2
Fedora 17
mlt-0.7.8 (I think. It's the most recent Fedora version from the repos/rpmfusion but I'm not able to check right now)
Hardware is quad core i7 2.2GHz, 8GB RAM, Nvidia GT550M (Nouveau).

If anyone is able to help me with this it would be greatly appreciated!