A _observable is delayed or works in real time? or is similar to app_idle_observable ( 100ms or 150BPM & 4LPB ) ???
Everything other than app_idle_observable and timers is very “sequential” and fully predictable.
At least in 99.99% of all cases. I extracted info about the sandbox in the formula device via parameter value observables, and remember i had to make the process slower to catch all characters. That was a very special case, though 
Regarding line strings, you can have a look at my revision of ledger’s “Set track width to active columns” - https://forum.renoise.com/t/new-tool-2-7-3-1-set-track-width-to-active-columns/31078
It uses pattern matching on line strings, which can be a lot faster than having to iterate note columns.