That vsync off was wrong picture, in that there`s AFK limitter just went on…
Here is new image with correct fps
System info
I couldn`t replicate this in flat world, placed over 4K sand blocks. I re-created my world in creative mode and this happened after I placed just few sand blocks.
After that I loaded my world and managed to record this video
This is happened in local singleplayer game
Here`s a video. You can see player hand movement changes too when it slows
This is similar situation, only that drowned is partly under ice.
Version 26.2