If you want to know more what RuleEngine etc., means:
https://github.com/eclipse/smarthome/issues/5040
The safeCall thread pool mainly controls the concurrency for handling commands and state updates.
Every ThingHandler#handleCommand and ThingHandler#handleUpdate is run through the SafeCaller in its own Thread.
The ruleEngine thread pool controls the concurrency for rule execution, basically how many rules can be executed at the same time.
For tasks which are scheduled from a ThingHandler (like script execution from the Exec binding or periodic polling) there is a separate thread pool thingHandler which can be configured.
Every day I learned one more things.
Now I have no more delays on my rules.
THX