This problem has been something that I have had since using 1.20.6. In 1.20.6 I would use Sodium and Optifine and I would get these lag spikes that would affect my game and overall make it terrible to play, but normal Minecraft would not have those lag spikes. With this I assume that it would be the same for 1.21, but when I logged in to play today it would do lag spikes and be really bad. I was playing on a realm that was updated for 1.20.6 to 1.21. I was wondering if it was me not allocating more ram and that didn't fix the issue. So I monitored my RAM and CPU, nothing changed or seemed to be out of the norm for what it always has done. I even uninstalled Minecraft and reinstalled it but that didn't fix it at all. With that I turned off a Nvidia setting that would turn off threaded optimization which I saw in a reddit post helped them, but it did nothing. With this I was wondering if this was a Minecraft issue or my PC, so I decided to load up a old update 1.18 and test to see it did the same fps drop and to my surprise it didn't do that at all. So now I'm stumped on what to do since I tried everything I saw online and still don't know why its doing this.