Downloaded flat worlds lag A LOT with no reason.

I wanted to play on a flat world, but I needed more depth, then I downloaded a world with 64 blocks deep. There was no mob or entity, but there was so much lag! And I felt it while trying to build. I tried with a 32 blocks deep one, and the problem persisted, even though there was absolutely nothing.

Here's the comparison with a Standard Flat World.

Please solve this, thanks