I added some simple metrics logging to the mod to see how many civs are ever alive at the same time. The highest I have seen is up to 32 in the 19th century up to the Canadian spawn. It will likely be a bit more until 2020, but I only ran one game until then. But also, for most of the game it never goes beyond 25 active civs at the same time. I also noticed that often there are 6-10 civs alive that are past their historical fall day, so if stability rules were stricter there could even be less civs active.
Consider that the game currently has 50 player slots that are theoretically available, with the corresponding performance costs. Looks like this can be cut down to 40 without issues, and even lower if it was more aggressive about collapsing civilizations to "make room". I could even see a world where the game is forced to make do with no more than 25 slots.
Since most parts of the logic have already been changed to be civ instead of slot based, this is now not that hard to do anymore. Replacing scenarios with custom maps will finally introduce a dynamic assignment of civs to the next free slot, which will allow reducing the number of slots overall and should bring a significant performance gain.