The current way the server works(I think) is every tick the game checks all "fully loaded" chunks for blocks that need updating, such as fire, crops, saplings, etc.
Steps for firetick would be something like:
Does fire spread?
Where does it spread?
Can it spread there?
Does fire extinguisher?
This can lagg the server alot since nothing else can update at this time. (Just explaining for people who might not know)
If this tick was handled on a separate thread the resources would likely take more RAM, but with that you free the rest of the server up.
A downside I can see is that more fire could make fires last longer just because it would have to wait the same time as if the server was lagging due to fire. That being said it would free up resources for other things.
This ought to be applied to not just fire, but Mobs and plant growth as well.
Implementing this would be great and I 100% support this. This could also help improve lagg from changes in light levels as well.