That is helpful, thanks. I actually didn't expect it would be more than 50% and suspect it may have had to do with the cutoff time being where it was because unless I am missing something that is how it should be.
How many pops did the planet have in the end in both cases? Mostly wondering for the purposes of estimating a ballpark percentage value for the amount of labour you miss out on.
So how syncretic grows is that at after the first month a growing pop for both species is created and they both grow perfectly evenly until they hit the target number of new pop creation. For the following let's call the two races the "masters" and the "slaves". What happens then is, to the best of my knowledge and in this order:
1) The slave race is selected for growth (maybe it's random when they tie but for me it's always been slaves)
2) A new slave is instantiated, that is their growing pop is replaced with a fully grown pop
3) The target number for all growing pops in raised in accordance with there being one more pop on the planet.
4) A new growing slave pop is created with 0 progress (if there is space)
5) Growth split is recalculated, in this case the balance of pops switches from 4:4 to 5:4 in favor of slaves.
6) Growth is applied according to the new split. This means the total growth now goes 5/9 towards the newly created slave pop and 4/9 to the mostly grown master pop. Before growth was split evenly.
What this results in is that two pops grown sequentially are actually faster than two pops grown in parallel, because once the slave finishes growing it reduces the growth of the master until the split is 50:50 again. This effect is relatively minor if you start out with 8 pops and perfect balance right away, but consider a planet with just one pop of each race. First they grow equally, then one instantiates and growth split changes from 50:50 to 67:33 until the second is finished growing. Granted, the target number also doesn't rise as much with going from 2 to 3 compared from 8 to 9.
There's also to consider that master-slave planets are behind (compared to sequential) until both races have grown at least one pop. This means if you start of with something like an 7:1 split then you're basically down some ~12% growth speed until the planet is full, because that's about as long as the single race will require to grow it's pop. So going out of balance is hard to avoid and actively bad.
So that is why being behind 50% of a pop is a lower bound to syncretic evolution. Two races perfectly balanced is the best case scenario for multispecies empires. Having 3 or 4 biological races growing on a planet at the same time stunts growth even worse for much of the same reasons, I don't remember the exact results but I once measured how long it would take if you started with 1 pop of 4 species and waiting until each species grew at least once. I do remember that it was significatly worse than sequential growth. That last pop just takes forever, starting out with 1/4 and slowly going down to 1/5, 1/6 and eventually 1/7 of the total growth available.
TL;DR: I'm glad we're getting a new growth system that hopefully allows for migration pacts and multicultural empires without impacting growth speed.
