Hey,
Check out this article about a new algorithm for faster previews. Maybe its possible to implement in corona? Wdyt?
We are aware of this. 
Meanwhile, here is a poor-man’s version:
Important note: this is just a toy made by AI. It can definitely crash, so please do not use it with production scenes without a backup. It may also lead to other issues that I am not aware of.
Download script: autoregion-v07.zip (5.9 KB)
Basically, it only updates the selected object(s) in IR, and only starts rendering the whole frame after a given pass/time limit. This way, you get faster feedback on the object you are editing.
Note: the pass/time limit works differently than in Corona - it always waits for both limits to be reached. E.g. if time limit is reached first - region rendering will stop only once the pass limit is reached. Have fun!
Demo video:
