Vlad
Right there with you, and slowly progressing to gather these! (I'm a developer as well, so know how annoying it can be when the right info and insights are not available.)
To preface this report, I have to say that the tricky part here is "how long is enough", as the CPU load seems to be tied to how long time the Mac has been allowed to sleep. This is also what's going to make a reproduction instruction tricky. So, for easy verification I left the Mac over night, giving it close to 10 hours.
As per request, I "replicated" my Orion setup by opening each and every page I had in Safari (BTW, is there any built-in function to gather all of the URLs open in various tabs, or maybe even a tool to quickly open up all open pages in Safari? This could speed up quite a bit).
As for Extensions, I disabled them all and then restarted Safari. Is this enough, that the extensions are still installed but disabled from loading? Also, same question goes for Orion, as I guess this will one of the next tests.
The part where this test becomes a bit more "synthetic" is that for each of the 45 tabs opened I simply entered the URL and loaded the page. I did not fully log in to the individual pages and services.
Then, I shut down Orion, and left the Mac with Safari open over night.
When I logged in a few minutes ago, I went straight in, no sluggishness on the lock screen, and no noticeable slow down in MacOS in general. 
Screenshot attached shows a minimal peak just after logging in.
So, as for way forward, I see two options as for what to do over night:
- Refine the "synthetic" Safari test, by logging in to the pages, making any background activities tied to accounts behave closer to Orion.
- Disable the extensions in Orion and see if that has any effect.
As per the question above, is it enough just to disable them from loading from the Extension management dialog, or do they even need to be uninstalled?
As for reproduction, I will revisit this after the two above has been tested. Maybe @rosenrot has done some additional attempts for reproduction since the initial post?
