Something went wrong in installation, software (apps), or hardware, 10.14.4 is typically as fast as prior OSX versions, even on old hardware (like ~2012 or 2013 Mini with 6Gb RAM). My first suggestions would be (make sure you have backup first!):
1. Disk utility, check drive. May be something is not right with disk structure. Disk errors will cause this easily. And they can develop any time.
2. Reinstall Mojave. Sometimes something does not install right. Reinstall in place (without wiping the drive) can help - and should be non destructive.
3. Backup drive to external drive (CCC is great for this) and wipe the drive, install Mojave system, and test. If all works fine, try migrating your data/applications back using Migration Assistant. If is slow again after migration, you have something bad/incompatible in your apps/data. In which case you can wipe again, install Mojave, and then migrate only data you need manually and reinstall applications you need - making sure your apps are Mojave compatible.
Downgrading is bit painful so try these steps first, may be it will help. Good luck.
PS: Encryption on its own does not slow down system for users - at least observably. I encrypt all of my drives and while they clock bit slower when encrypted, it is not observable in real life. ~10% throughput reduction of read/write speed I have tested consistently is not observable for users most of the time and not responsible for beach balling.
