diff --git a/Spigot-Server-Patches/0370-Optimize-Biome-Mob-Lookups-for-Mob-Spawning.patch b/Spigot-Server-Patches/0370-Optimize-Biome-Mob-Lookups-for-Mob-Spawning.patch
new file mode 100644
index 0000000000..9f560569b7
--- /dev/null
+++ b/Spigot-Server-Patches/0370-Optimize-Biome-Mob-Lookups-for-Mob-Spawning.patch
@@ -0,0 +1,80 @@
+From 12b790f4486e8c965077d12175aafeb535bd9388 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Wed, 12 Sep 2018 21:47:01 -0400
+Subject: [PATCH] Optimize Biome Mob Lookups for Mob Spawning
+
+Uses an EnumMap as well as a Set paired List for O(1) contains calls.
+
+diff --git a/src/main/java/net/minecraft/server/BiomeBase.java b/src/main/java/net/minecraft/server/BiomeBase.java
+index 66c038784c..b6990a6d7a 100644
+--- a/src/main/java/net/minecraft/server/BiomeBase.java
++++ b/src/main/java/net/minecraft/server/BiomeBase.java
+@@ -118,7 +118,7 @@ public abstract class BiomeBase {
+     protected final Map<WorldGenStage.Decoration, List<WorldGenFeatureComposite<?, ?>>> aW = Maps.newHashMap();
+     protected final List<WorldGenFeatureCompositeFlower<?>> aX = Lists.newArrayList();
+     protected final Map<StructureGenerator<?>, WorldGenFeatureConfiguration> aY = Maps.newHashMap();
+-    private final Map<EnumCreatureType, List<BiomeBase.BiomeMeta>> aZ = Maps.newHashMap();
++    private final java.util.EnumMap<EnumCreatureType, List<BiomeBase.BiomeMeta>> aZ = Maps.newEnumMap(EnumCreatureType.class); // Paper
+ 
+     @Nullable
+     public static BiomeBase a(BiomeBase biomebase) {
+@@ -155,7 +155,7 @@ public abstract class BiomeBase {
+             }
+ 
+             for(EnumCreatureType enumcreaturetype : EnumCreatureType.values()) {
+-                this.aZ.put(enumcreaturetype, Lists.newArrayList());
++                this.aZ.put(enumcreaturetype, new MobList()); // Paper
+             }
+ 
+         } else {
+@@ -297,7 +297,7 @@ public abstract class BiomeBase {
+     public void a(WorldGenStage.Decoration worldgenstage$decoration, ChunkGenerator<? extends GeneratorSettings> chunkgenerator, GeneratorAccess generatoraccess, long ix, SeededRandom seededrandom, BlockPosition blockposition) {
+         int jx = 0;
+ 
+-        for(WorldGenFeatureComposite worldgenfeaturecomposite : (List)this.aW.get(worldgenstage$decoration)) {
++        for(WorldGenFeatureComposite worldgenfeaturecomposite : this.aW.get(worldgenstage$decoration)) { // Paper - decompile fix
+             seededrandom.b(ix, jx, worldgenstage$decoration.ordinal());
+             worldgenfeaturecomposite.a(generatoraccess, chunkgenerator, seededrandom, blockposition, WorldGenFeatureConfiguration.e);
+             ++jx;
+@@ -459,6 +459,38 @@ public abstract class BiomeBase {
+ 
+     }
+ 
++    // Paper start - keep track of data in a pair set to give O(1) contains calls - we have to hook removals incase plugins mess with it
++    public static class MobList extends java.util.ArrayList<BiomeMeta> {
++        java.util.Set<BiomeMeta> biomes = new java.util.HashSet<>();
++
++        @Override
++        public boolean contains(Object o) {
++            return biomes.contains(o);
++        }
++
++        @Override
++        public boolean add(BiomeMeta biomeMeta) {
++            biomes.add(biomeMeta);
++            return super.add(biomeMeta);
++        }
++
++        @Override
++        public BiomeMeta remove(int index) {
++            BiomeMeta removed = super.remove(index);
++            if (removed != null) {
++                biomes.remove(removed);
++            }
++            return removed;
++        }
++
++        @Override
++        public void clear() {
++            biomes.clear();
++            super.clear();
++        }
++    }
++    // Paper end
++
+     public static class BiomeMeta extends WeightedRandom.WeightedRandomChoice {
+         public EntityTypes<? extends EntityInsentient> b;
+         public int c;
+-- 
+2.18.0
+