Games starts lagging to unplayable levels without mods which is a major problem...
I've ran into that. It turns out that the problem was with how my kindles OS hogs resources and keeps a million programs running in background. Which combined to create an unstable pplatform. Try parking a bunch of stuff that doesn't need to be hanging around in standby mode.
Also you haven't given any info about: the number of competing nations and nation states; the size of the map; what your system is; how many turns in you were when your system became unstable; exactly wtf is lagging, (eg. While the computer players are taking their turns, to calculate auto moves,) ; which animations you have turned on; are you playing multiplayer or single; how old your device is, etc.