Single Player "Received passengers for unknown entity"

My single player world that has never had anyone other than myself connected has started immediately repeatedly spamming the message "Received passengers for unknown entity" as soon as I enter the Nether. Soon after the message "Can't keep up! Is the server overloaded? Running 2019ms or 40 ticks behind". Frame rate drops and eventually becomes unplayable.

 

THIS interface is very problematic.

 

The error occurs when running the game completely vanilla, and also if running OptiFine alone with no other mods. The error disappears when I leave the Nether. I've tried running command "/kill @e[type=!player]" while in the Nether and 460 entities were killed, but the issue remains.

 

This issue is not the same as other similar issues. I have never played multiplayer in this world. No other player other than myself has ever been in this world.