I’m not absolutely sure about this. But I noticed (in my RaspiPi) that if a rule is currently processed, all other rules have to wait until this is finished. Maybe this is valid only on the same rules file. I can’t say how good OH is on multithreading. But it relies on an event system and each event is normally processed one by one.
Since I noticed this, a optimize everything and try to schedule (timer) as much as I can and completely prevent hughe calculations if not necessary. It looks like timers are running in it’s own thread and don’t block anything.
Please (real) OH pros. Can you bring light into the darkness? 