I just got a new macbook pro that has Mojave 10.14.6 installed, and I'm running Final Cut 10.14.6
It worked fine for about a day, but when I opened a library today it will load fine, but wont actually play anything. I get the rainbow wheel for a while, then the viewer will show the option to pause (like it is playing) but nothing plays - and anytime you move the playhead or try and open any panels it just gets the spinning wheel for a while then doesn't do anything.
Besides what FCPX.guru said, verify you are not running the Chrome browser. There is a known incompatibility between newer versions of Chrome and FCPX. This is caused by Chrome monopolizing the VideoToolBox framework, thereby locking out FCPX and causing lags and hangs.