1.17-1.21.1(Forge/Fabric) Pre-release version are accessable through Patreon:

A complete overhaul of Minecraft's performance issues. It drastically improves serverside performance, which is most notable in the form of slow entity movement and blocks taking long to drop. Affects multiplayer and singleplayer, as singleplayer is using an integrated server.

Most notable features:
Less AI overhead

AI's no longer spent like half their time deciding which task they should run

Multithreading of entity collisions and pathing

The expensive parts of entity updates, collisions pathing and movement are faster and happen multithreaded.

FPS

Inbuilt entity-culling stops entities from rendering if they're not visible.

Login/playing timeout

Configurable login timeouts for logging in and disconnecting while playing, helping players with slower PC's and connections.

Recipe caching

Recipe lookups get a lot faster, helping with all those autocrafting mods out there causing lag when looking through the vast amount of recipes.

Fast world updates

Random block ticks, chunk ticks and mob spawning are faster overall and cost less performance.
Fixes:

Leashing lag, lag caused by leashed entities.

Hopper lag, those only act when required now.
Advancement lag caused by checking inventories
Entities loading chunks when they should not

and many more smaller ones you can find in the configs.

All features are configurable and can be tweaked/enabled/disabled in the config.

Set entity density, increase or decreasing the spawning of all entity types
Enable dynamic entity simulation distance
Limit scheduled block updates of e.g. Water flowing to a certain amount per tick
Forbid certain entity types from existing in the world, useful for entities causing issues

Can be installed on either side, does not have to be on both. Improves Performance on both client and server, feel free to use it in any modpack

All the performance changes are done in a way which is compatible with most mods out there, only known to be incompatible are:
Lithium(and forge ports of it: Roadrunner, Canary)

WYML
Those are incompatible, as they have a similar aim to performant so modifications do overlap.

If you enjoy this mod consider supporting the development, any support is greatly appreciated.

The development is an unpaid free-time activity, so any support helps being able to spend more time on it.
You can support me on Patreon, by using the bisect server code above or just spreading the word around.

Commands:

starts recording lag data:

/performant startGatherData

Reports lagging entities and stops recording:

/performant reportEntities

Reports lagging tileentities and stops recording:

/performant reportTileEntities

Reports any currently slowed down event subscribers
/performant reportEvents

Performant