Hi first time poster here. Currently running Final Cut Pro X 10.4.6 on Mojave 10.14.4 and getting many beach balls with the audio skipping. This could take upto a minute to rectify but then the same problem will happen. Editing 4K content but the same thing happens whether using the original media or proxy media.
Running the 2018 Macbook Pro i9, 32GB RAM, Vega 20 and SSD drive. Even editing the content on High Sierra on a 6 year old Macbook Pro didn't have this many issues.
My internal SSD is 500GB with around 300GB free and I run my libraries and media from a 1TB external SSD. I never had this issue on High Sierra and noticed that others have had it when they updated to Mojave 10.4.3 but the same problem still seems to be in 10.14.4. Are you running Mojave to? I have cleared render files etc several times but the problem persists.