There are lots of lags in 1.21.1. It often drops to 0 fps for a few seconds, and I often can't cancel my inputs, this might cause falling from a high place/into lava.
This happens less in 1.21.
My render distance is 12 chunks.
My PC performance is shown in F3 screen in the video.
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Maybe, idk

This may be MC-273024, since you are generating chunks in your video. Does the lag occur while crossing a chunk border?

Seems not: it occurs at X:587
Your integrated Intel Iris Xe Graphics are having a hard time running Minecraft as it looks like. Does lowering your render distance and simulation distance help or does it still lag? I would also suggest enabling VSync or at least limiting your fps.

It still lag but much less.
Would MC-273262 describe your issue?
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I think it does now.
Would MC-273262 describe your issue? Lag problems are very general and can have any number of causes.