World generation and rendering regressions in 1.14

In 1.14 world generation is extremely slow and rendering issues often occur. This is seems to be more noticeable in Linux but is also an issue in Windows as well. Standing around in an improperly rendered chunk can result in other chunks becoming invisible. Once the world generates previously generated invisible chunks become visible again. In 1.13 this was never an issue.

It seems to also be possible to corrupt game saves sometimes by saving your game near an invisible chunk and reloading the world where then you will eventually get a concurrent modification exception. 

To reproduce: set view distance to 32 chunks and in creative keep flying in chunks that have yet to load. Previously loaded and rendered nearby chunks will disappear and reappear.