I played around with it a bit, and it seems like the lag is directly proportional to
1. how many lines are in the maps for each Incest Mode, and
2. how many lines are on the screen.
Full Incest is the laggiest, Sister is better, and Aunt (empty) is lag-free. The mod could be optimized, but you can get rid of the UI lag even without that. The lag is most severe in the save menu and URM because there's more text on the screen. The in-game text box and choice menus only have a few strings on the screen at a time as you play, so the lag is mostly unnoticeable, but the save menu and URM can have dozens of strings on each screen. The mod goes through all the hundreds of replacement lines from the active maps for every single string on the screen and tries to replace them before it loads.
You should make it so that the mod ONLY tries to replace the text inside the text box and the choice menus, NOT any of the UI at the bottom of the screen or the save menu or the preferences or URM or anything else. The fewer irrelevant strings it tries to replace, the better it will perform.