The bug
Lag Spike is occurred every 3 seconds when open the world. Not entirely sure what caused this but I think it's bug with rendering system. Affect all graphic settings.
Step reproduce
Create New World (Normal or Flat)
Press Alt + F3
See the FPS graph with lag spike
Have a at the Task-Manager while this happens, does anything out of the ordinary happen, and can you see any significant differences between 1.16.1 and 20w28a?