diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -44,6 +44,7 @@
   usersWithPersonalThreadSelector,
 } from '../selectors/user-selectors.js';
 import { getUserAvatarForThread } from '../shared/avatar-utils.js';
+import SentencePrefixSearchIndex from '../shared/sentence-prefix-search-index.js';
 import type { CalendarQuery } from '../types/entry-types.js';
 import { messageTypes } from '../types/message-types-enum.js';
 import {
@@ -1773,6 +1774,42 @@
   ]);
 }
 
+function useChatMentionSearchIndex(): {
+  +[id: string]: SentencePrefixSearchIndex,
+} {
+  const { chatMentionCandidatesObj } = useChatMentionCandidatesObjAndUtils();
+  return React.useMemo(() => {
+    const result = {};
+    for (const threadID in chatMentionCandidatesObj) {
+      const searchIndex = new SentencePrefixSearchIndex();
+      result[threadID] = searchIndex;
+      for (const key in chatMentionCandidatesObj[threadID]) {
+        searchIndex.addEntry(
+          chatMentionCandidatesObj[threadID][key].id,
+          chatMentionCandidatesObj[threadID][key].uiName,
+        );
+      }
+    }
+    return result;
+  }, [chatMentionCandidatesObj]);
+}
+
+function useThreadChatMentionSearchIndex(
+  threadInfo: ThreadInfo,
+): SentencePrefixSearchIndex {
+  const chatMentionCandidatesSearchIndex = useChatMentionSearchIndex();
+  const { communityThreadIDForGenesisThreads } =
+    useChatMentionCandidatesObjAndUtils();
+  if (threadInfo.community === genesis.id) {
+    return chatMentionCandidatesSearchIndex[
+      communityThreadIDForGenesisThreads[threadInfo.id]
+    ];
+  }
+  return chatMentionCandidatesSearchIndex[
+    threadInfo.community ?? threadInfo.id
+  ];
+}
+
 export {
   threadHasPermission,
   viewerIsMember,
@@ -1844,4 +1881,5 @@
   getThreadsToDeleteText,
   useChatMentionCandidatesObj,
   useThreadChatMentionCandidates,
+  useThreadChatMentionSearchIndex,
 };