Modpack Performance Optimization Guide

Before You Start — Identify What Kind of "Lag" You Have

What people call "lag" is actually two different problems mixed together. Low FPS and low TPS (Ticks Per Second) have different causes and need different fixes. Press F3 in-game and you'll see both numbers in the top-left corner — check this first, otherwise you'll waste hours fixing the wrong thing.

Low FPS (typically below 30) — Your GPU/CPU can't keep up with rendering. Fix with in-game video settings + disabling shaders.

Low TPS (below 20) — Server-side simulation is falling behind. Caused by mod conflicts, chunk leaks, or overloaded automation. Fix with JVM settings and chunk cleanup.

Both low — Almost always insufficient RAM or a Java version mismatch.

Spend the first 5 minutes *checking F3 to see which side has the problem*. Everything else comes after.

JVM Argument Settings — The Heart of Lag Fixing

The single biggest factor in modpack performance is JVM (Java Virtual Machine) configuration. A lot of players only touch in-game settings and leave JVM at defaults — but tuning JVM properly can add 5–10 TPS to the same modpack.

RAM Allocation — Too Much Is Also a Problem

Lightweight modpacks (50 mods or fewer): 4 GB

Medium modpacks (100–200 mods): 6–8 GB

Large modpacks (200+ mods): 8–12 GB

ATM10 / StoneBlock late-game: 12 GB recommended

⚠️ Allocating too much RAM backfires: GC (Garbage Collection) cycles get longer, freezing the game for 1–2 seconds at a time. Allocating 12 GB on a 16 GB system also slows your OS down. *Never exceed 60% of your PC's total RAM.*

Recommended JVM Arguments (G1GC-based)

-XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200
-XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC
-XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40
-XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20
-XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4
-XX:InitiatingHeapOccupancyPercent=15 -XX:G1MixedGCLiveThresholdPercent=90
-XX:G1RSetUpdatingPauseTimePercent=5 -XX:SurvivorRatio=32
-XX:+PerfDisableSharedMem -XX:MaxTenuringThreshold=1
-Dusing.aikars.flags=https://mcflags.emc.gs

This is known as Aikar's Flags — a battle-tested G1GC tuning set used by Minecraft servers. Paste it into the CurseForge launcher under instance settings → "More Options" → JVM Arguments (alongside -Xmx/-Xms for memory).

Matching Java Versions

1.21+ NeoForge modpacks → Java 21 required

1.20.1 Forge → Java 17

1.18–1.19 → Java 17

Pre-1.16.5 → Java 8 (or Java 17 with compatibility)

*Wrong Java version = guaranteed crash.* The CurseForge launcher matches versions automatically, but on Prism Launcher and similar tools you have to set this per-instance manually.

In-Game Settings — Squeezing Out FPS

Video Settings (Top 4 Priorities)

Render Distance: 8–12 chunks. *16+ will lag in late game, no exceptions.*

Simulation Distance: 6–8 chunks. This is *heavier than render distance.*

VSync: ON (reduces unnecessary GPU heat, eliminates screen tearing)

Particles: Minimal. Once your automation lines grow, particles explode and tank FPS.

Additional Recommended OFF Settings

Clouds: OFF

Entity Shadows: OFF

Smooth Lighting: Minimum (or OFF)

Biome Blend: 0 (OFF)

Squeezing Even More from Specific Mods

Dynamic Surroundings: Disabling weather/sound effects can give you 30–50 extra FPS

Better Foliage: Turn off leaf detail

Particle Man / Particle Culling: Limit particle render distance

Embeddium / Sodium: If your modpack already includes it, enable everything under the *Performance* category

Chunk Management — Late-Game TPS Fixes

In late game with heavy automation, you'll often see *FPS staying fine but the game stuttering anyway*. That's a TPS problem caused by too many loaded chunks.

Common Causes of Chunk Leaks

Leaving Mekanism Digital Miner running while traveling far away: Its chunk loader keeps memory occupied

Multiple Create windmills / waterwheels: High CPU usage (heaviest among automation mods)

AE2 ME systems with large auto-craft queues: Long queues validate every tick

Multiple chunk loader mods in use: Often loads more permanently than intended

Fixes

F3+T to instantly reload chunks/resources (lightweight emergency fix)

Disable unused automation lines with Redstone

Install a *profiler mod* like Spark or Observable to see which mod/block is eating TPS (server-side, recommended for multiplayer)

In late game, the lightest setup is letting natural *player-adjacent chunk loading* do the work instead of permanent loaders

Shaders — What to Know Before Turning Them On

Shaders make the game gorgeous but *cost you 30–50 FPS instantly.* Recommended approach:

Play the first 60 hours without shaders — get familiar with the modpack itself first

Start with light shaders: *Complementary Reimagined* (included by default in Better MC)

Heavier shaders (BSL, SEUS, etc.) only when you have GPU headroom in late game

Iris (for Fabric) or Oculus (for Forge) is the shader loader

*Distant Horizons* combined with shaders is too heavy — use one or the other, not both

Crash Troubleshooting Basics

1. How to Read a crash-report

Go to your instance folder → crash-reports/ and open the latest file in a text editor.

---- Minecraft Crash Report ---- Time: ... Description: Watching Server

java.lang.OutOfMemoryError: Java heap space at net.minecraft.world.chunk... (Mekanism: ProcessChamber...)

The key is finding the error type + the mod name in this kind of structure.

2. Common Errors and What They Mean

OutOfMemoryError → RAM shortage. Increase your -Xmx value

NoSuchMethodError → Java version mismatch (e.g., a Java 17 mod running on Java 8)

NullPointerException at modX → Configuration or compatibility issue with modX. Disable that mod's config

TickTimeoutException → A specific mod is taking too long per tick. Usually overloaded automation

3. Step-by-Step Resolution

Find the mod name in the crash-report

In that mod's folder (config/{modid}/), disable the suspicious option

If that fails, *delete that mod alone* and try launching again (verify compatibility with other mods)

Last resort: report on the modpack's Discord or the "Issues" tab on CurseForge

Restore from your backup save

Recommended Measurement Tools

F3 (in-game debug): FPS, TPS, RAM usage, chunk count

Spark (mod): TPS profiling, shows which code/mod is heavy

JEI search bar: Search by mod name to track which mod added which item