Even in my small base of 4:10 hours, performance drops to 20 with 2.XX in debug view from expansion mod, and 60 stable when disabled. Looking at the code I think maybe assign chunks that have trees to reproduce every minute with ln(n) (while n < 700 to prevent chunk having dense forests ???) where n is numbers of trees at that chunk. I think even if trees were condensed at the center we would still have trees at the edge to spread to other chunks instead of being locked in the center. The rest of your tree mods are fine. nonetheless.
I did not test it heavily to reflect on it but I like the idea you are implementing, hoping it to be a great mod to change gameplay, while having configurable settings to adjust how dangerous someone likes the biters to be.
