The purpose of the Improved Fire Performance Mod is to boost game performance by removing redundant checks when a fire is started.
It does not affect the way fire burns or spreads. As explained by Dawgydude:
“RimWorld runs in ticks; at x1 speed it does 60 ticks per second. Every tick, the game runs code on every dynamic object on the entire map (e.g. pawns, plants, animals, and of course fire) in order to update that object. So if there are 500 blocks of fire on the map thanks to a dry thunderstorm, then it’s running fires tick code 30,000 times every second (500×60).
This mod slightly improves the performance of the fire tick code by moving code around, precalculating some values, and removing redundant checks. This means the code its constantly running is just a tiny little bit faster , which adds up to a massive performance boost given how often it needs to be run every second.
For machines with weaker CPUs (or those players with tons of mods running), you shouldn’t see as much of a massive FPS drop anymore when there are lots of fires on the map.”