SkyBlock Performance Tuning — Sodium, Iris, JVM Args (1.21.11)

PERFORMANCE · March 12, 2026 · 9 min read

SkyBlock performance tuning: Sodium, Iris and the heap settings that matter

Short version. Sodium gives you 80–100% FPS. FerriteCore cuts heap by 30%. Lithium smooths server tick. The JVM args below have been the test target for every ASR release for the last six months and they hold up under 30+ mods.

The stack we test ASR against

Sodium 0.6.2 — rendering rewrite. Mandatory.
Iris 1.8.5 — only if you run shaders.
Indium 1.0.30 — Sodium compat layer if you mix in other rendering mods.
Lithium 0.13.x — server-tick smoothing, ~5% TPS.
FerriteCore 7.0.x — deduplicates block-state references. ~30% heap reduction past 20 mods.
EntityCulling 1.7.x — culls offscreen entities. Noticeable in Garden + Crystal Hollows.
MoreCulling — additional culling for repeated blocks.

JVM args we ship with

These are the args the official launcher Fabric profile uses out of the box, with three tweaks. Drop them in your launcher's "JVM Arguments" field.

-Xmx4G -Xms2G
-XX:+UseG1GC
-XX:G1NewSizePercent=20
-XX:G1ReservePercent=20
-XX:MaxGCPauseMillis=50
-XX:G1HeapRegionSize=32M
-XX:+UnlockExperimentalVMOptions
-XX:+ParallelRefProcEnabled
-XX:+AlwaysPreTouch
-XX:+UseStringDeduplication

Heap floor (-Xms) at half of max is important — touching the memory upfront avoids the G1 paging hitch during the first dungeon boss fight.

How much heap do you actually need?

Vanilla + ASR: 3 GB max, 1.5 GB initial
+ Skyblocker + Sodium: 4 GB max
+ NEU + 20 other mods: 6 GB max, 3 GB initial
30+ mods + shaders: 8 GB max, 4 GB initial

More heap is not better. Past 8 GB on G1GC you start to see longer pauses because the collector has more to scan. If you're hitting OOM at 8 GB you have an actual leak, not a sizing problem.

Diagnosing FPS drops

Open F3 — read FPS, then look at Memory %.
If Memory % is >85, your heap is too small (or you have a leak).
If FPS is fine but TPS is hitching, that's server-side; nothing client can fix.
Toggle ASR off (/asr disable) and recheck. Delta >5 FPS, file an issue with latest.log.
Toggle Sodium off (rename the jar) and recheck. If FPS drops 50%, Sodium was working — keep it.

Shaders

Complementary Reimagined is the SkyBlock-tested choice in 2026. BSL works but has visible flickering on Crimson Isle ash particles. SEUS Renewed is heavier than CR with no SkyBlock-specific benefit.

Shader expectations: expect a 30–40% FPS hit on Complementary. If you can't pay that and stay above 60 FPS, don't run shaders during dungeons — the timing reads matter more than the prettiness.

OS-level

Use Java 21 (the one bundled with the official launcher works fine).
Windows: set Minecraft to "High Performance" in graphics settings if you have a dGPU.
Linux: disable VSync if you're on a Wayland compositor; X11 doesn't need this.
macOS: native ARM build of Minecraft is meaningfully faster than the x64 build on Apple Silicon.

Related: /performance/ · Best settings