Put the summary of the bug you're having here

What I expected to happen was

that the game would finally run smooth (it has been lagging since 1.13 but not as badly)

What actually happened was...:
I created a super flat world and i started getting 15 FPS (Frames per second)

Minecraft used to run fine when i played version 1.12.2

Steps to Reproduce:
1. Generate a super flat world.
2. Exit the world and re-enter the world.
3. You will find out that it lags a lot.(or is it just my PC)