This has been updated for 26.1 - 26.2, all of the optimization mods mentioned here are compatible with each other and don't affect vanilla behaviour by default.
Lithium - A mod designed to drastically improve the general performance of Minecraft without breaking things.
VMP - A mod designed to improve general server performance at high playercount.
C2ME - A mod designed to improve the performance of chunk generation, I/O, and loading. This is done by taking advantage of multiple CPU cores in parallel.
ScalableLux - A mod that rewrites the light engine to fix lighting performance and lighting errors.
FerriteCore - A mod that reduces the memory usage of Minecraft in a few different ways.
Krypton - A mod that optimizes the Minecraft networking stack and entity tracker.
Fast Noise - A optimization mod to improve world generation times that maintains vanilla parity, including with any datapacks. It optimizes storing block and biome data into chunks for the duration of worldgen.
RecipeCooldown - A mod that prevents excessive crafting recipe request from the client to lag out the server. (The 26.1 version should work on 26.2 as well.)
Structure Layout Optimizer - A mod that optimizes general Jigsaw structure generation. This mod aims to not change how structures look at all, it only works to try and make structures generate faster and more efficiently.
Disable Portal Checks - Disables unnecessary portal checks that causes lag.
RailOptimization - A simple mod that makes powered/activator rails turn on/off up to 4x faster. Also fixes vital crash.
Chunky Pregenerator - A mod which pre-generates chunks, quickly and efficiently.
Spark - A performance profiler mod.
View Distance Fix - A mod that fixes the fog when having a higher render distance on the client than on the server. The server tricks the client into thinking that it has the same render distance as the client.
quick pack - A mod that optimizes datapack / resourcepack zip file loading times.
Ledger = A logging mod similar to CoreProtect.
Carpet - A mod for vanilla Minecraft that allows you to take full control of what matters from a technical perspective of the game.
Carpet Extra - A mod that adds extra features to the Carpet mod.
sync-chunk-writes=false
max-chained-neighbor-updates=10000
simulation-distance should ideally be 9 if you want all types farms to work as intended, but lowering it will improve performance
Since 26.1 Minecraft requires Java 25 to run. If you don't have Java 25 installed already you can get it from here.
Generate a start-up script for your server here.
If you're willing to try out a more modern garbage collection method then check out my set of Java start-up flags.
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a small suggestion, dont you can lock conversation thread (issue/pr/etc) in github? if you want to forget this?