bradywurtz wrote:
....I've even got Youtube in the background playing videos and they'll freeze up too, so it's the whole computer locking up. sometimes my mouse freezes...
If the entire machine and OS is locking up, it's usually not the fault of FCPX or any other user-mode app. If a non-privileged app could lock up the entire machine, a malicious version of that could take over the system.
Run Apple Diagnostics on the machine. No errors does not mean a "clean bill of health" as it's a simple test. However if it finds something it can be significant:
support.apple.com/en-us/HT202731
Rebuild Spotlight indexes on each volume, as documented here:
support.apple.com/en-us/HT201716
You can also check what kernel extensions are loaded. They run in privileged mode and can lock up the system if they mis-behave:
osxdaily.com/2010/08/03/list-all-third-party-kernel-extensions/
Verify each hard drive is delivering expected performance by running Black Magic Speed Test on each one. If a hard drive is intermittently failing or doing re-tries, that can disrupt the system.
If your H264 media is 4k, that can be slow on a 10-core iMac Pro much less a 2013 iMac 21.5. If they are 4k you will need to create proxies for good editing performance. If they are 1080p, performance should be OK without proxies or optimized media, but you could try optimized media. Note optimized media is about 6x the size of H264 originals so disk I/O rates will increase by about 6x. This decreases CPU load at the cost of increased I/O load.