Twixtor in FCP workflow

Hello,
I finished all the tutorials and read the manual for twixtor in FCP I understand the concept of the plugin and must admit that the results are sometimes really stunning.
The only problem I have is how to incorporate this valuable plugin in to everyday editing work in FCP. Being in a timeline with different clips, here and there there is a need to slow down,time remapp a clip, with the possibility to review quickly and make further adjustments, for example to match music ....
Although I think that I have a strong hardware setup...MacPro 3.ghz, Ati 1900, 6 gb ram, the rendering process is awfully slow (hdv footage) which makes it almost impossible to use it as a editing decision tool. Of course, if I decide to change a whole clips speed, than there is no problem, I just make a longer coffee break, but what if I would like to make accurate speed adjustments that can be easily reviewed. I just hope that there is something I don't know about the use of twixtor inside of FCP. RT doesn't work even if I switch between safe and unlimited (as someone suggested ) So if anyone who uses Twixtor could give me an advice or his workflow inside FCP, that would be really great
Thank You

Piotr Janowski