I have a 2013 Mac Pro 10.10.2 running on Yosemite. I upgraded the RAM to 64GB(4X16) and it doesn't seem to be much faster while processing FCPX files. Are there any settings I can check to ensure it's running at top speed?
Processing files requires CPU/GPU power. More ram helps by caching the files in memory, but your bottleneck probably is the CPU (or maybe a slow storage, if you're using external disk).
Also upgrading to the latest macOS version could help, since FCP X is using Metal, and Metal 2 is incorporated in High Sierra.