There are big performance issues with Minecraft 1.14.
A lot of lag spikes appear when the game loads and unloads chunks.
The generation is really very slow and some chunks do not even appear at all.
Sometimes, chunks can not charge at all or can flicker.
We have the impression that we came back in 2018 with the first snapshots of the 1.14 with the new rendering engine!
How was it done to regress at this point? Because it was not too bad in 1.14 Pre-Releases...
1. Create or load a world in 1.14
2. Once in play, press Alt+F3 to display the lagometer graph
3. Explore your world and keep an eye on the graph
Here are examples in videos: [to come up]