Old game crashing after uncovering most of the map

I made a world especially for when my nephews play with me. I've already uncovered most of the map, have all of the winter's feast, midsummer's cawnival, year of, and hollow nights event sections made for them. And a base that they will never run out of food or items (they are to lazy and I don't want them in my usual worlds because I want to make it past spring). But after I unlocked most of the map they haven't been able to login. At first it would lag near the waterlogged biome, then when I would try to switch to specific characters or try to change their skins. Now my nephews can't even login, and if they do not for long. The new world I made without exploring the whole map works fine, but I don't have it as advanced as the other one. And can't get there because I don't what to keep exploring just to have it start to crash like last time.