I don't know how to describe it exactly... I've been transferring villagers one by one through the nether, and then everything seemed to lag and speed up at random intervals, but only in the overworld. Villagers would freeze, then zip around. I checked the debug menu and the "ticks ms" switches between ~7 and >300 every couple seconds. I've tried relogging, reloading, going back through the nether... Nothing fixes it.
I do keep backups of my worlds, so I'll only lose the work I've done today.