FCP not using Full CPU

because it's not programmed to use all cores . . . sadly

 

Get used to it. FCP isn't very multi-thread aware.

The real advantage of having multiple cores with Final Cut Studio is when using Compressor for encoding. With a proper QMaster virtual cluster set up, it can bring your CPU cores down to their knees if you want it to.

I wouldn't render straight out of an FCP timeline or even send a timeline straight to Compressor. Set up a virtual cluster in QMaster first. Export the video from the timeline as a self-contained QT movie and then drop that file into Compressor. When you submit the batch, choose the name of the cluster you created in QMaster and you'll observe quite a difference in encode times.

 

Lots of operations are also heavily HDD dependant so that might also bottleneck...