Creatures Caves

There are at least two major possibilities I can think of that could be causing this.

-First question.... How many exported creatures are in your My Creatures folder?
The game will lag horribly if there are too many exported creatures in that game. I've had the game take several minutes just to load, save, export or import. The fix for this is to move some of the creatures into other folders to get them out of the My Creatures folder. The game won't read any folders inside My Creatures, it only reads the base folder. So making folders inside My Creatures to store your creatures will work just fine and let the game load at normal speed.

-Second question, or rather, second set of questions.... Has this high gen population been in this same world for a lot of generations? Do you just save the world with the population still in it, or do you export the population every so often and move them to a new world?
If they have been in the same world for a very long time, with lots of generations being born and dying there, then.... it possible that the problem may be too many genetic files or image files in folder for that world. Go to the My Worlds, then find the folder for the world that is having issues. Take a look in the Genetics folder, the Images folder & the Basement folder. See how many files are in those three folders. The game will save the genetic files and the custom sprite sets of each creature that has been in that world to those folders. Over time these can really add up and start lagging the game. I've really only had the game lag from this when I let the High Gen Generator run for several hours or overnight. It is possible to delete the old genetic files and sprites to speed the game back up. That's what I did to fix the lag. But you have to be careful if you clean them out of the Basement folder, don't remove anything that isn't a genetic file or sprite file. Before trying this method to fix it, if possible export your creatures to keep them safe, or copy and back up the world just in case something goes wrong while trying to fix it. I don't know what will happen if you delete the genetic file of a creature that is still in the world, so be careful and delete only the genetic files for the creatures you know are dead or gone. I think the game will just regenerate the image files if they are deleted.

Hopefully this helps.