Setting the Density Factor to negative values causes extreme lag when the world is loaded. I think it was caused by that.
Fun Fact: It generates the noise upside down.
When I loaded it, my frames fell down to 0 - 1.
MC-209311
Setting the Density Factor to negative values can cause extreme lag when the world is loaded
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If it generates the world upside down, I'm guessing the lag is the game trying to process a few hundred thousand water updates every tick.