I deeply regret having swapped out my late 2012 maxed out iMac for the new 5K Retina....Final Cut Pro X has been EXTREMELY laggy and very buggy. I crash every few minutes....My advice, avoid the 5K iMac until next year when all these problems are resolved...In FCPX, our timelines are about 4 hours long with 6 to 8 layers of media. We have six 8TB GRAID drives running on two separate Thunderbolt chains.
Sorry about your difficulties, but there is no fundamental problem with the 5K iMac or FCPX. I have 64 TB of Thunderbolt storage on my 2015 iMac and have an FCPX library containing 8,085 clips (181 hr of 1080p and 4K material), comprising 4.75 TB total size -- all in a single event.
I usually have shorter timelines, however prior versions of FCPX were tested with a timeline up to 588 *days* long, and with up to 1,000 layers on a Mac Pro:
http://www.fcp.co/final-cut-pro/new...w-mac-pro-and-final-cut-pro-x-to-their-limits
Here is a complete walkthrough of a seven-camera professional event that was recently edited on FCPX. They shot one terabyte per day using Sony field production cameras. FCPX was used to edit a four-hour, seven-camera timeline, of which they had three different versions open:
I have had problems with FCPX crashing due to corrupted video files, which you will often have when aggregating a huge amount of content from various sources. In this case it was damaged MTS which someone had extracted from the AVCHD bundle and cut up with a 3rd-party clipping tool. When I isolated and removed those, the crashes ceased.
I also had some Event Browser performance problems from bare MTS files. I removed these and re-wrapped them with EditReady, then re-imported them and the problems went away:
http://www.divergentmedia.com/editready
There is no basis to say avoid the 5K iMac until next year, nor any basis to believe that all problems will be resolved then.
However it might be a good idea for anyone planning on large video projects to wait until the next iMac 27 update -- not because the current one is flawed but because the next one will be faster and video (esp. H264 4K) is very demanding.
Apple just released FCPX 10.3 which supposedly has some further performance updates but I can't test it until I am finished with my current project.