in macOS mojave apps freezes and beachball appears for some seconds when doing tasks like loading a conversation in Messages
This is easy to isolate.

Sign out of Messages and make sure no other app or system is causing the slowdowns. The more you get freezes, the quicker you can know that it’s just messages causing the slowdown as opposed to being impacted. YOu might open Activity Monitor and show all processes to see which are using a lot of CPU.

Then you can make a new user account. In that account - only sign in to messages - see if the slowdown is there (assuming you concluded it was the cause above).

If so - then you might check with Apple Support to be sure you don’t have an issue server side and possibly consider backing up and restoring your Mac to make sure it’s not part of the system that’s stuck or causing the slowdown.

Keep in mind, for physical hard disks - the first sign you get of imminent failure can be slow apps, so maybe make a good backup first and then test to isolate what’s slow.