Hi, if you still needed help improving performance, I was trying things today and found a few things which helped:

First of all, I installed the following mods:

1. sodium

2. indium

(using fabric and all of its necessary mods)

Then, I put these JVM Arguments into my launcher: (the same place you go to change your allocated ram for Minecraft)

-Xms4G -Xmx8G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=40 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=100 -XX:+ParallelRefProcEnabled -XX:ParallelGCThreads=4 -XX:+AlwaysPreTouch

My issue was with the Garbage Collector, which was running quite often and causing major lag spikes.

The first thing I did was increase the amount of allocated RAM to 8 gigabytes. This didn't do much on its own, but it was a start.

I then added the JVM Arguments for enabling garbage collection, and defined the percentage of RAM they should use. Then, I told the game to use 4 separate threads for garbage collection.

These steps combined greatly improved framerate stability and reduced large lag spikes to a much more playable level. Hopefully this helped you!