From 21w19a (and 21w20a) I experience extrame low, unplayable 1-2 fps in the game with high res (512px) resource pack. Prior to this version I had no issues with it (27 fps in 21w18a and below).
I'm trying to make a high res resource pack (512*512 blocks), and it worked fine until this last two snapshots. If I turn off the resource pack everything is fine again, no performance problems.
I played with these options which were working until now:
-Xmx8G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
I cannot decide if this is some bug or I just need to do some new settings for these new versions/client.
Attached the screenshot of the debug windows in version 21w18a and 21w20a to see the differences (with the resource pack on).
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But my problem is it had NO performance issue at all in 21w18a and prior versions. But in the last two snapshots (with the java 16 changes) it has. I'm creating a resource pack for 1.17, that is why I want to use always the latest snapshot.
I made video about them as you asked:
21w18a https://youtu.be/Hb6-rrtJJms - it runs playable, as it should
21w20a https://youtu.be/6tQmpH6pdh4 - it drops to 1-2 fps, unplayable
Duplicate ofย MC-219639.

Thank you for your report!
We're tracking this issue in MC-219639, so this ticket is being resolved and linked as a duplicate.
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Well, thank you, I will check that ticket. I used the search function before but since I didn't know what to look for in my case, it was impossible to find that ticket.
Just play without the resource pack or with a lower resolution one, as the highest the resolution, the lower the game's performance. If you could upload a video showing how the resource pack affects the game performance, it would help a lot.