I've made some test around other seen i've read in this thread.
My last project was a "bigger" one starting from M2T files from my Z5 (converted as i mentionned yesterday). The final project time is 15 minutes and i added some Red Giant FX. My usual export options was (in french : Dispositifs Apple 720p). I didn't let FCPX do render in background.
After 24 hours, i was at 40% of exportation.
Then, i choose the option 'Render Master' that i never chosse before because my project will be only uploaded in Vimeo.
The result was realy better, it takes me 8 hours. But i reported some entrelacement disagrement in the master file.
On my second mac, i let it do backround rendering during some times. I can't measure it, but was sensibly quicker than 8 hours ! After that when all the background rendering was finished, a Render Master only take few minutes !