Great news! I did what BellaClo suggested, as well as turned down my graphics settings to medium, and both UC and my dynasty file are running perfectly, albeit in separate folders - I just rename the folder I don't want to play and everything runs fine, no lagging at all. I suspect the combination of a very large dynasty file plus UC is what caused my original problem.
Anushka, this was your first world?! I'm absolutely amazed at that - it's so perfect! I hope you and the rest of the team don't get discouraged by all of us asking for technical help - it in no way reflects on your work or how much all of us love and appreciate it. You guys did something that I bet EA wishes they could have done, and you did it brilliantly. Now you all are handling Help Desk functions brilliantly. I hope you and the team don't think that posts about difficulties some of us are having are in any way meant as complaints, because I think nothing could be farther from the truth.