JEIOptimizer

Builds JEI's search index in parallel — cuts server join time by up to 17 seconds on large modpacks.

The story

I was playing on a private AllTheMods server with friends. Every disconnect, every restart, every "let me grab a snack" meant 40 seconds of staring at "Joining world…" while everyone else was already mining.

Got tired of it. Spent a weekend with JEI's source code open instead of playing the game. This mod is what came out.

What it does

When you connect to a modded server, JEI rebuilds its entire item search index from scratch — because the server may ship custom recipes. On a 250-mod pack with ~50,000 items, this takes 12+ seconds on the main thread. Your client just sits frozen.

JEIOptimizer builds that index across worker threads instead. Each search prefix (item name, mod ID, tag, …) owns an independent data structure, so they can be built at the same time — no contention, no races.

Real-world measurements

Tested on a 250+ mod pack (AE2, Mekanism, Create, Cobblemon, Apotheosis, AllTheCompressed, …). Ryzen 5600X · 10 worker threads · NVMe · 8 GB RAM allocated · remote server (Hetzner DC).

Combined with the tooltip tweak below, server connect drops from 40 s → ~23 s consistently.

How it works (technical)

JEI's search index is split into "prefixes" — name, mod ID, tag, tooltip, creative tab, colour, resource ID. Each one owns a separate suffix tree, which makes them safe to build independently.

Vanilla JEI: one thread walks every ingredient for every prefix, in sequence.

JEIOptimizer: the prefixes are built at the same time, one per worker thread.

The tooltip prefix is deliberately kept on the main thread. Building it calls ItemStack.getTooltipLines(), which runs arbitrary mod code — and plenty of mods touch main-thread-only state there. It still runs concurrently with the worker pool, so nothing is lost in wall-clock time, but nothing gets called off-thread that shouldn't be.

Filter modes (in config)

• OFF — vanilla JEI behavior
• BATCH — small cache-friendly improvement, single-threaded
• PARALLEL_PREFIX — parallel per-prefix build (safe)
• PARALLEL_FULL — also parallelizes tokenization within a prefix (default, fastest)

Recipe generation

JEI builds a number of synthetic entries at startup by driving a hidden menu once per combination — one entry per enchantable item × enchantment × level, plus repair entries for every damageable item in the pack.

Enabled by default (transparent): writing each input slot used to trigger a full recipe recalculation, and the first one could only ever see half the inputs. Those redundant recalculations are now skipped. Same entries, same values, less work.

Optional (changes what you see): the generated entries can be skipped entirely. On packs with hundreds of enchantments this is a large startup win, but those entries disappear from JEI. Off by default — real recipes are never affected either way.

[recipes]
    skip_redundant_menu_updates = true        # transparent, nothing is lost
    skip_generated_enchantment_recipes = false
    skip_generated_repair_recipes = false

Free 8 more seconds (manual JEI tweak)

In config/jei/jei-client.ini, change:

tooltipSearchMode = DISABLED

JEI indexes the tooltip text of every item on every connect. Some mods do heavy work in tooltip rendering — eating a large share of those original 12 seconds for nothing. Players almost always search by name, not tooltip text.

If you still want tooltip search via the $ prefix, use REQUIRE_PREFIX instead.

Configuration

config/jeioptimizer-client.toml (auto-generated):

[filter]
    mode = "PARALLEL_FULL"   # Default. Safe, fastest.

worker_count = 0             # 0 = auto (cores - 2)
log_timing = true            # Log timings for verification

[plugins]
    parallel_phases = []                # Experimental — see below
    parallel_creative_tabs = false      # Experimental — see below

[recipes]
    skip_redundant_menu_updates = true
    skip_generated_enchantment_recipes = false
    skip_generated_repair_recipes = false

Safe defaults out of the box. Drop the jar in, you get the win. No config touching required.

Compatibility

• Minecraft: 1.19.2 · 1.20.1 · 1.21.1 · 26.1.2 · 26.2
• Mod loader: Forge (1.19.2, 1.20.1) · NeoForge (1.21.1 and newer)
• JEI: matching build for each version — see the file listing
• Side: Client-only — server installation not needed

JEI changes its internals between minor releases fairly often, so each build targets a specific JEI range. Grab the file that matches your pack.

What this WON'T speed up

Being honest about scope:

• ❌ Game launch (mod construction, model baking, resource pack reload)
• ❌ Server data sync (recipes, tags, registries) — network and mod serialization bound
• ❌ Per-mod runtime data regen — Create generates 2,124 recipes every connect, Cobblemon resyncs all fossils/marks/berries, Apotheosis reloads all affixes. These are mod-design choices.

JEIOptimizer removes JEI from the bottleneck. It's no longer the long pole — other mods still do their thing.

Experimental features (default OFF)

Tested but found to break things on real mod packs. Available as opt-in for those with carefully chosen mod sets:

• plugins.parallel_phases — Parallel plugin recipe registration. Many JEI plugins (Theurgy, Ars Nouveau, Compact Machines) are not thread-safe and crash.
• plugins.parallel_creative_tabs — Parallel creative tab build. Many mods cache tab state non-thread-safely; in our test 22% of JEI items were lost.

Leave these off unless you know what you're doing.

FAQ

Q: Will this break JEI search? Default settings change how the index is built, not what. Same items, same recipes, same search results.

Q: I have a smaller modpack. Will I see less benefit? Yes. Below ~10,000 items the gain is marginal. Designed for 30k+ item packs.

Q: Why not submit a PR to JEI? Maybe later. Mezz (JEI's author) historically prefers conservative changes — parallelism in core code may not be welcomed. This mod exists so you don't have to wait for upstream.

Q: Server-side install needed? No. Pure client-side mixin. Putting it in mods/ on a server is a no-op.

Q: Does it work with EMI / REI? No, this targets JEI specifically. EMI has its own architecture (and is already fast).

Credits

Tested on a custom 250+ modpack and AllTheMods10, AllTheMons.

Thanks to Mezz for keeping JEI's source open and readable enough to optimize from the outside.