Pretty heavy iMessage user here, but I can't say I experience any issues, and that's probably why your issue is not getting fixed - ie. nobody at Apple is able to reproduce it. Maybe you should gather some info about it and see if you can send a bug report?

It happens more often when switching devices, particularly if it’s a less than regularly used device like an iPad. Happens with my travel MacBook. Takes messages a solid day to catch up.

The most I’ve heard back from a reproducible bug report was ”cool, it shouldn’t do that”. The response came on the dev forums, the actual bug has never been acknowledged or fixed. Multiple times. Why bother?

What I've found is if I open a picture in iMessage it tends to trigger the CPU hungry behavior. I notice it after a while as my laptop starts getting hot and battery draining much faster than expected. I hard quite iMessage, reopen it, and all is fine.

fair point, I should — one classic symptom I experience is the emoji picker will make it crash, not load quickly, and if it finishes loading they all appear as empty placeholders (maybe because I have way too many stickers and iCloud sync is tripping? idk)