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).