Lithium is a mod for Minecraft

What Lithium does

Lithium speeds up Minecraft’s internal computations: game logic, block updates, entity processing, and other tic operations.

What exactly is being optimized

Lithium works deeper than the graphical interface: it changes the implementation of individual calculations of the game engine, preserving the usual mechanics of Minecraft. Its domain is tic processing, entity renewal, physical and logical operations. When farms are running on the map, mobs are moving around, and multiple checks are performed at the same time, the cost of such calculations becomes especially noticeable.

Client and server

On a dedicated server, the point of Lithium is to free up the time of the main game stream so that tics are executed without undue delay. In a single-player game, part of the logic is also executed locally, so the effect is possible without a server. Direct changes in the image should not be expected: FPS and TPS measure different aspects of performance. If you’re running unstable, first figure out which subsystem is loaded and check for compatibility with other optimization modes.

What to compare on the server

The most understandable indicator for server logic is the tick execution time at a comparable load. If players complain about delays in opening containers and mob reactions, check the MSPT and TPS, not just the client's frame rate. On a small blank map, the differences can be almost unnoticeable, so test conditions close to the usual peak online.

Primary source: developer-page.