This bug starts on Snapshot 25w09a and is still present on 1.21.5 rc 2. Usually in game, I get around 110-120 FPS on my Microsoft Surface Pro (Snapdragon X Plus model).
In this snapshot and onwards, I get around 40-50 fps.
I am running the game at the same resolution and the same settings on both versions.
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No. I tested the same seed on both versions when I realized the performance felt off.

It’s not because of the new falling leaves either because I tried one of the pre 25w09a versions that had falling leaves and it ran fine. It’s specifically 25w09a and above.

Just tested this using the same world on an M4 processor between 25w08a and 25w09b. The fps while flown up at 32 chunks had actually improved for me on 09b, though sometimes it seems to be near error. I also did not encounter any lag spikes, including whilst flying around.
Enough swing to be in error, but generally positive.
A noticeable improvement. The camera/height adjustment did not affect the fps here.

So I guess that means that 1. Minecraft made some change to the rendering engine between the snapshots and 2. This may be a Windows on ARM exclusive issue.

I’m not having lag

Yes sandbox this does not affect x86-64 only ARM

I think this and MC-295893 are indeed separate issues as ARM64 devices clearly note a much more severe performance regression than x86 machines.

Have the same issue with newest version 1.21.5, after the update on my Realms world I have very low FPS in some places (even below 30). I play on my Macbook Pro M1 2020. It used to be over 100 fps all the time.

This was mysteriously fixed on my device in a later snapshot of 1.21.6 even though I haven’t seen anything about it in the changelog and this hasn’t been marked as resolved.
I now get fps in the 110-120 range again.
Were there any specific terrain features that could impact performance?