a lag appears, if ocean monuments and mansions can not generate in the world and the fourth trading-tier of a cartographer is reached.

reproduction:
1. create a world in which no ocean monuments and mansions can generate.
2. spawn a white-robed villager; a cartographer.
3. trade his first three offers to unlock the fourth tier in which you should be able to get the two explorer maps.
4. exit the trading GUI. the lag appears in a brief moment. in the end the fourth tier will still be locked.

little sidenote: not being able to reach the fourth tier makes the villager quite unattractive, i think it would be nice if an alternative to the explorer maps would be implemented, if these maps can not be obtained.