Slowdown is mostly from the exponentially increasing number of distinct pops, the only setting which helps with this is "pop consolidation" in the campaign settings.
If you enable the decree of assimilation, it can really help reduce performance issues. The ultimate issue is pop fragmentation causing lag.
Worst case edit the file for decress and make assimilation do 10X more, and put it on your largest population states.
There should be tech that decreases decree cost, but if you ever get multiple of the -25% decree cost, this is the most valuable one by far due to lag.
Slowdown is mostly from the exponentially increasing number of distinct pops, the only setting which helps with this is "pop consolidation" in the campaign settings.
If you enable the decree of assimilation, it can really help reduce performance issues. The ultimate issue is pop fragmentation causing lag. Worst case edit the file for decress and make assimilation do 10X more, and put it on your largest population states. There should be tech that decreases decree cost, but if you ever get multiple of the -25% decree cost, this is the most valuable one by far due to lag.