FCPX extremly slow on 1st gen MacPro

I have a 1st generation Intel Quad Core MacPro, 20 GB of RAM, Nvidia Geforce 5770 HD, 12 TB of drive space, and since I can't upgrade my system any more Lion 10.7.8 I believe (or whatever the latest is). All media are on separate internal 2TB WD-Caviar Black drives, and media has been optimized to ProResLT. FCPX is setup to NOT background render, and waveforms have been turned off in the media browser.

Over this past weekend I was doing some editing on a project and FCPX was extremely non-responsive.
The timeline has always been a little touchy in that the play cursor never runs perfectly smooth when playing the timeline.

However, when ever I even pressed the playbar to start/stop, it took about 3 seconds for FCPX to respond.
Also if I even tried to select clips, or trim, slip, etc., it took FCPX several seconds to respond. I will say that last night I turned off the waveforms on the storyline and FCPX was a little more responsive. Not much a bit more.

Needless to say this drove me nuts and made my editing much more time consuming and not nearly as productive as it normally has been. I will say that I have been editing with a FW800 drive in my office on a 3.47 Ghz iMac running Mountain Lion, and everything responds fine.

Anyone have any ideas on what the issue might be, as there's no way I can work like this on a regular basis.
Could the issue be NOT running Mountain Lion and running Lion on my older system?

I am going to upgrade my system to a new iMac, but not for a few months yet.

Thanks,
Michael

I know nothing of the Nvidea Geforce 5400, but I'm guessing that is where your problem is.

I don't see it listed as a compatible card.
support.apple.com/kb/HT4664

I run a 2008 Mac Pro with an ATI Radeon 5870 with 16 GB RAM and internal SATA drives. It handles REDCODE Raw native files better than my brand new 15" rMBP maxed out on an SSD Tbolt drive.

Sorry typo on my part the graphics card should be the Nvidia 5770 HD.

You say "all media is on the external drive," but what about the actual FCPX Project files? You need to make sure your settings in FCPX stipulate that everything to do with the project is saved & written to that external drive. (Also, I'm just assuming you know that external drive needs to spin at 7200 RPM or faster.)

Another thing that I've found has worked for me on various Macs, is streamlining my software. I run Monolingual thru everything on my system (it removes all the extra languages I'm not using). Then I run XSlimmer, which removes many gigs of useless code that equips the software to run on other Macs, not mine. It's been my experience that when all this superfluous code is excised, my system drive has several more gigs of space AND more importantly, everything, yes ... EVERYTHING runs faster & smoother.

Thanks for the feedback.

All of my media and EVENT and PROJECTS files are on separate internal 7200 WD-Caviar Black drives.

I have XSlimmer so I'll give that a try. I never heard of "Monolingual" but I'll look that up and give that a try as well.