diff --git a/static/ui.js b/static/ui.js
index 0d4abf6b..c8249f8b 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -265,10 +265,15 @@ function _statusCardHtml(card){
 const MESSAGE_RENDER_WINDOW_DEFAULT=50;
 let _messageRenderWindowSid=null;
 let _messageRenderWindowSize=MESSAGE_RENDER_WINDOW_DEFAULT;
+// Cached visWithIdx array — invalidated when S.messages.length changes.
+let _visWithIdxCache=null;
+let _visWithIdxCacheLen=0;
 function _resetMessageRenderWindow(sid){
   _messageRenderWindowSid=sid||null;
   _messageRenderWindowSize=MESSAGE_RENDER_WINDOW_DEFAULT;
   _clearRenderCache();
+  _visWithIdxCache=null;
+  _visWithIdxCacheLen=0;
 }
 
 // ── renderMd / _renderUserFencedBlocks cache ──────────────────────────────
@@ -6120,15 +6125,29 @@ function renderMessages(options){
     ? (()=>{const row=document.createElement('div');row.innerHTML=`<div class="compression-turn"><div class="compression-turn-blocks">${_compressionReferenceCardHtml(referenceText,false)}${_preservedCompressionTaskListCardsHtml(preservedCompressionTaskMessages)}</div></div>`;return row.firstElementChild;})()
     : null;
   let preservedCompressionTaskCardsAttached=!!referenceNode;
-  const visWithIdx=[];
+  // Cache visWithIdx so expanding the render window (Load earlier) doesn't
+  // re-scan S.messages from scratch.  Invalidate only when the message array
+  // length changes — i.e. new messages arrived or session was truncated.
+  if(!_visWithIdxCache || _visWithIdxCacheLen !== S.messages.length){
+    const rebuilt=[];
+    let ri=0;
+    for(const m of S.messages){
+      if(!m||!m.role||m.role==='tool'){ri++;continue;}
+      if(_isPreservedCompressionTaskListMessage(m)){ri++;continue;}
+      const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
+      const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
+      if(msgContent(m)||m._statusCard||m.attachments?.length||(m.role==='assistant'&&(hasTc||hasTu||_messageHasReasoningPayload(m)))) rebuilt.push({m,rawIdx:ri});
+      ri++;
+    }
+    _visWithIdxCache=rebuilt;
+    _visWithIdxCacheLen=S.messages.length;
+  }
+  const visWithIdx=_visWithIdxCache;
   const preservedCompressionRawIdxs=[];
   let rawIdx=0;
   for(const m of S.messages){
     if(!m||!m.role||m.role==='tool'){rawIdx++;continue;}
     if(_isPreservedCompressionTaskListMessage(m)){preservedCompressionRawIdxs.push(rawIdx);rawIdx++;continue;}
-    const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
-    const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
-    if(msgContent(m)||m._statusCard||m.attachments?.length||(m.role==='assistant'&&(hasTc||hasTu||_messageHasReasoningPayload(m)))) visWithIdx.push({m,rawIdx});
     rawIdx++;
   }
   // Show a top affordance when earlier transcript content exists either in