Slow User Interface Performance on a Mac Pro

Question

I've recently purchased a late 2013 6-core Mac Pro with 16gb of memory and the 256gb SSD.  While the general performance of Final Cut Pro X is outstanding (applying filters, transitions, analyzing for dominant motion, optical flow retiming, rendering, etc.), the user interface seems to still start to slow down the further along I am in my project.  It is most noticeable when I click a clip in my timeline to modify a video property in the inspector, then hit play (spacebar) again to resume editing.  At the start of a project, I can fly through my clips and edit without any hesitation, but as I near the 90 second-ish mark (with an edit/clip every 5-10 seconds), the responsiveness of the user interface starts to waiver just like my previous early 2011 Macbook Pro (something I thought was related to the graphics and/or system performance of computers from that era).
I've tried creating proxy and optimized media projects and even edited my h.264 footage in native, compressed format - regardless of what I try, the performance of the user interface is nearly identical until I start to hit a few minutes with some colour corrections on 25% of the clips.  I've also experimented with the playback quality during editing (i.e., best performance vs. quality) which resulted in minimal performance improvements.
Clearly, this Mac Pro has been designed to tear through video footage and process mass amounts of data, but I wonder if there is something else looming in the code of Final Cut Pro X that, regardless of the computer in use, the user interface just gets sluggish at a certain point when a critical mass of edits/clips are in a project.
Any comments or feedback would be greatly welcomed, as I may decide to return this beast and settle on an iMac if this symptom exists regardless of what Mac I use.
Sincerely,
Ryan Smith

Answer 1

1.  All of my media was located on the SSD installed inside the Mac Pro and is the startup disk of the computer
This will slow you down.  Get a secondary drive for the media and Libraries.
2.  The SSD had between 50g and 150g (depending on the project) available
That drive is becoming too full, and slowing down.
3.  The project consisted of a single event from one import, about 100 files totalling 40g
A good USB 3 or Thunderbolt external RAID will fly through that without issue.  The system drive will get bogged down.
4.  I haven't paid close enough attention to Activity Monitor; I will start doing that, but FCPX is the only application I ever have open when editing and consumes about 12gb - 16gb.
16GB RAM should be plenty for FCPX.  I've never seen it eat up that much RAM, even on very large projects.  But more RAM won't hurt if the system is using that much RAM.  How are you getting those RAM usage numbers?
The issue seems to be your drive.  First, don't use the system drive, that will bog things down, no matter what type of drive it is.  Second, get your media and Libraries on a fast secondary (external) drive. 
Finally, are you running with the dual D500 or D700 GPUs?  There is a significant performance difference between the two in FCPX and Motion 5.  But I highly doubt that's your issue.  I think you simply are eating up too much of your drive, and running off the system drive, both are huge no-no's.