It is hard to tell what exactly is happening here. I think that "longer" messages may need to be redrawn causing the layout to be changed (changed item size) but that is done by macOS under the hood so I cannot fix that, however, it may be also caused by sending "chat markers" to self to mark messages that were read (syncing read status between clients), but that shouldn't take so long.

Could we maybe pre-render all the unread messages upon opening particular chat? Going all the way down and then back to the "Unread messages" maker kinda fixes the issue and one can read the thread/messages without any jumping/jolting.

I do not think we can, as prerendering is done by macOS when required. It is possible to prerender everything, but as it is time and CPU intensive it is disabled by default. I've tried to do that some time ago and it made Beagle basically useless due to the time required to open the chat.