Block random ticks

Most blocks in the game perform random ticks, and this part is quite optimized in itself. Yet, the more blocks you have, the bigger the strain on a server. Even stairs do randomTick() for some mysterious reason. Preventing randomTick() is a somewhat safe option. You just prevent some mechanics from running, like copper weathering or bamboo growing. At least in vanilla, I haven't found any breaking blocks that crash the game if they are banned. I have randomly selected 50% of all the blocks in the game and banned them. No issues found.

Block ticks

This territory is full of surprises. Some blocks may be unaffected at all, while others may stop working or only half of their functionality may be gone. E.g., minecraft:fire: if you ban it from ticking, the only thing that will happen is it won't destroy blocks (if set on wood, for example) or extinguish. So, basically behaves like on netherrack. While furnaces don't give a shit. So, unless you know how a block works or are ready for a long process of trial & error, I recommend avoiding this feature.

Fluids

Since version 0.3 of the mod and 1.21.1 of Minecraft, you are able to prevent fluids from ticking and random ticking. Be careful with that option. Fluids without ticks just stay like a liquid block forever, but they will interact with the world and neighbors. Vanilla has only two liquids, water and lava, so my tests are somewhat limited. Only lava has a random tick method, and all that it does is set things on fire. You can toggle that behavior in the gamerules, so random tick prevention may be useful only for liquids from other mods. Also remember that fluids have at least two types: flowing and normal.

Debug

Since version 0.3 of the mod and 1.21.1 of Minecraft, I've added the option to display on load and reload of the config what was blocked and what failed.

Usage

The way I see it, this mod may help you:

Squeeze some performance if you are in a really tight spot. This is for a server, remember; this mod helps the server way more than the client.
Prevent some mechanics from happening while leaving the block or fluid.
Debugging

If you want to see a performance comparison, see readme.md on the mod's GitHub page.