shaking camera while looking up from bottom of world or down from top of world causes big FPS drop when vsync is ON while playing in windowed mode.
To reproduce create void world, stand on platform at Y=-60, look up and move slightly your mouse. On RTX2080ti while playing in small window FPS drops from 80-90 to 20.
FOV 90, clouds off.
VSYNC on and windowed mode is required to reproduce problem.
Turning off vsync rises FPS from 20 to ~75.
Making game fullscreen with vsync ON rises FPS from 20 to ~70

Same behaviour when on top of world and looking down.
Render distance tested from 12 to 20 chunks.