This is an odd one. Using Neoforge version 1.21.1 (21.1.147) and creating a new modpack, I found a specific combination of mods that cause massive lag.

Installing just the following three mods causes it.

Samurai Dynasty, CosmeticWeapons, and Macaw's Biomes O' Plenty

Installing any two of those three causes no issues at all. But all three together and FPS 1.

I've opened an issue with CosmeticWeapons as well (since I have no idea which one of the three might need a fix). Samurai Dynasty doesn't have a method to open issues but I'll try to message the author.

No, I wouldn't think so, but removing it fixes the problem. It's very strange. Also, it does not occur on Forge 1.20.1 at all, which I forgot to mention before.

It's worth mentioning that this problem happens even with none of the Macaw's mods loaded, which means this mod maybe shouldn't do anything at all, but... it must be doing something?

Macaw's BOP with AddonsLib create all block for macaw's mods with loop for (which stop), so it's probably not this mod.

For Data reason Macaw's BOP generate all block for paths window or fences, but if Macaw's Fences is not installed, the fences is not added in creative tab.
Have you looked the ram allocated in minecraft launcher, for increase memory ?

I have 8 gigs allocated, which should be plenty. Memory usage sits at about 32% while running.

I had a crazy idea tonight, and decided to see what happens if you strip out all of Biomes O' Plenty and replace it with Oh The Biomes & Macaw's Oh The Biomes. Same exact problem.

Honestly, my first suspect for causing problems was Samurai Dynasty, given that it adds such a wide variety of armor, weapons, ores, structures, and I'm not even sure what else.

extreme_lag.log

Try to install LogBegone : https://www.curseforge.com/minecraft/mc-mods/log-begone/files/5609009

I don't think it affected the log file size much.

latest.log

I recreated your modpack and I disable Samurai Dynasty mod, the lag was stopped instantly.
I change Adorned (Curios) [Discontinued] to Curios API (Forge/NeoForge) : https://www.curseforge.com/minecraft/mc-mods/curios
The lag has been stopped, I also install GeckoLib because in Samurai Dynasty Web Project Page is marked to require : https://www.curseforge.com/minecraft/mc-mods/geckolib

You should also install Macaw's Mods, because Macaw's BOP isn't very useful on its own: https://www.curseforge.com/members/sketch_macaw/projects

latest.log

Aha, Adorned was the problem! You have found the fix, and for that I give you my immensely grateful thanks.

Yes, my real modpack has every Macaw's mod installed, (I was just minimizing to find which mods were causing the problem.)

Truly, thank you very much.