diff --git a/server/src/main/java/org/elasticsearch/cluster/metadata/ProjectMetadata.java b/server/src/main/java/org/elasticsearch/cluster/metadata/ProjectMetadata.java
index 24adeedd7366e..5ea0bada50946 100644
--- a/server/src/main/java/org/elasticsearch/cluster/metadata/ProjectMetadata.java
+++ b/server/src/main/java/org/elasticsearch/cluster/metadata/ProjectMetadata.java
@@ -46,6 +46,7 @@
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashMap;
@@ -1761,7 +1762,7 @@ static SortedMap<String, IndexAbstraction> buildIndicesLookup(
             if (indices.isEmpty()) {
                 return Collections.emptySortedMap();
             }
-            SortedMap<String, IndexAbstraction> indicesLookup = new TreeMap<>();
+            Map<String, IndexAbstraction> indicesLookup = new HashMap<>();
             Map<String, DataStream> indexToDataStreamLookup = new HashMap<>();
             collectDataStreams(dataStreamMetadata, indicesLookup, indexToDataStreamLookup);
 
@@ -1769,7 +1770,121 @@ static SortedMap<String, IndexAbstraction> buildIndicesLookup(
             collectIndices(indices, indexToDataStreamLookup, indicesLookup, aliasToIndices);
             collectAliases(aliasToIndices, indicesLookup);
 
-            return Collections.unmodifiableSortedMap(indicesLookup);
+            // We do a ton of lookups on this map but also need its sorted properties at times.
+            // Using this hybrid of a sorted and a hash-map trades some heap overhead relative to just using a TreeMap
+            // for much faster O(1) lookups in large clusters.
+            return new SortedMap<>() {
+
+                private final SortedMap<String, IndexAbstraction> sortedMap = Collections.unmodifiableSortedMap(
+                    new TreeMap<>(indicesLookup)
+                );
+
+                @Override
+                public Comparator<? super String> comparator() {
+                    return sortedMap.comparator();
+                }
+
+                @Override
+                public SortedMap<String, IndexAbstraction> subMap(String fromKey, String toKey) {
+                    return sortedMap.subMap(fromKey, toKey);
+                }
+
+                @Override
+                public SortedMap<String, IndexAbstraction> headMap(String toKey) {
+                    return sortedMap.headMap(toKey);
+                }
+
+                @Override
+                public SortedMap<String, IndexAbstraction> tailMap(String fromKey) {
+                    return sortedMap.tailMap(fromKey);
+                }
+
+                @Override
+                public String firstKey() {
+                    return sortedMap.firstKey();
+                }
+
+                @Override
+                public String lastKey() {
+                    return sortedMap.lastKey();
+                }
+
+                @Override
+                public Set<String> keySet() {
+                    return sortedMap.keySet();
+                }
+
+                @Override
+                public Collection<IndexAbstraction> values() {
+                    return sortedMap.values();
+                }
+
+                @Override
+                public Set<Entry<String, IndexAbstraction>> entrySet() {
+                    return sortedMap.entrySet();
+                }
+
+                @Override
+                public int size() {
+                    return indicesLookup.size();
+                }
+
+                @Override
+                public boolean isEmpty() {
+                    return indicesLookup.isEmpty();
+                }
+
+                @Override
+                public boolean containsKey(Object key) {
+                    return indicesLookup.containsKey(key);
+                }
+
+                @Override
+                public boolean containsValue(Object value) {
+                    return indicesLookup.containsValue(value);
+                }
+
+                @Override
+                public IndexAbstraction get(Object key) {
+                    return indicesLookup.get(key);
+                }
+
+                @Override
+                public IndexAbstraction put(String key, IndexAbstraction value) {
+                    throw new UnsupportedOperationException();
+                }
+
+                @Override
+                public IndexAbstraction remove(Object key) {
+                    throw new UnsupportedOperationException();
+                }
+
+                @Override
+                public void putAll(Map<? extends String, ? extends IndexAbstraction> m) {
+                    throw new UnsupportedOperationException();
+                }
+
+                @Override
+                public void clear() {
+                    throw new UnsupportedOperationException();
+                }
+
+                @Override
+                public boolean equals(Object obj) {
+                    if (obj == null) {
+                        return false;
+                    }
+                    if (getClass() != obj.getClass()) {
+                        return false;
+                    }
+                    return indicesLookup.equals(obj);
+                }
+
+                @Override
+                public int hashCode() {
+                    return indicesLookup.hashCode();
+                }
+            };
         }
 
         private static void collectAliases(Map<String, List<IndexMetadata>> aliasToIndices, Map<String, IndexAbstraction> indicesLookup) {