Fine-Structure is a casual-play 1.13.2 server that we use for testing purposes. Loading it up into 19w11a exposes a new severe performance problem. One thread dominates the CPU time and pegs the CPU at 100%. I used async-profiler to analyze this, and a screenshot is attached. The full profile in HTML form can also be found at "https://www.dropbox.com/s/33lqat9x3yzkdny/tree2.html?dl=0".
According to people from the Fabric project, this is villager AI. Rockenroll4life confirmed that there's a new AI engine.
We do have a fair number of villagers in one area. In 1.13.2, there were lag problems due to entity collisions. In 1.14, the collision problem is fixed, but now there's significant lag caused by villager AI processing instead.