diff --git a/packages/ui/src/components/message-section.tsx b/packages/ui/src/components/message-section.tsx
index 300d9872..38d828ce 100644
--- a/packages/ui/src/components/message-section.tsx
+++ b/packages/ui/src/components/message-section.tsx
@@ -325,14 +325,15 @@ export default function MessageSection(props: MessageSectionProps) {
   const selectedTokenTotal = createMemo(() => {
     const selected = selectedForDeletion()
     if (selected.size === 0) return 0
-    const segments = timelineSegments()
+    // O(n) pre-pass: aggregate chars by messageId once.
+    const charsByMessageId: Record<string, number> = {}
+    for (const seg of timelineSegments()) {
+      charsByMessageId[seg.messageId] = (charsByMessageId[seg.messageId] ?? 0) + seg.totalChars
+    }
+    // O(selected.size) lookup pass.
     let total = 0
     for (const messageId of selected) {
-      let charTotal = 0
-      for (const seg of segments) {
-        if (seg.messageId === messageId) charTotal += seg.totalChars
-      }
-      total += Math.max(Math.round(charTotal / 4), 1)
+      total += Math.max(Math.round((charsByMessageId[messageId] ?? 0) / 4), 1)
     }
     return total
   })
diff --git a/packages/ui/src/components/message-timeline.tsx b/packages/ui/src/components/message-timeline.tsx
index e7af2ef8..b578fc39 100644
--- a/packages/ui/src/components/message-timeline.tsx
+++ b/packages/ui/src/components/message-timeline.tsx
@@ -519,17 +519,19 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
   createEffect(() => {
     if (isSelectionActive()) {
       computeBadgeLayout()
-      // Deferred pass: tool segments become visible when selection activates,
-      // but they may need a layout pass before getBoundingClientRect is accurate.
-      requestAnimationFrame(computeBadgeLayout)
-      window.addEventListener("resize", computeBadgeLayout)
-      onCleanup(() => {
-        window.removeEventListener("resize", computeBadgeLayout)
-        if (scrollRafId !== null) {
-          cancelAnimationFrame(scrollRafId)
-          scrollRafId = null
-        }
-      })
+      if (typeof window !== "undefined") {
+        // Deferred pass: tool segments become visible when selection activates,
+        // but they may need a layout pass before getBoundingClientRect is accurate.
+        requestAnimationFrame(computeBadgeLayout)
+        window.addEventListener("resize", computeBadgeLayout)
+        onCleanup(() => {
+          window.removeEventListener("resize", computeBadgeLayout)
+          if (scrollRafId !== null) {
+            cancelAnimationFrame(scrollRafId)
+            scrollRafId = null
+          }
+        })
+      }
     }
   })
 
@@ -548,26 +550,29 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
   const liveSegmentChars = createMemo(() => {
     if (!isSelectionActive()) return {} as Record<string, number>
     const result: Record<string, number> = {}
-    const processedMessages = new Set<string>()
     const resolvedStore = store()
 
-    for (const segment of props.segments) {
-      if (processedMessages.has(segment.messageId)) continue
-      processedMessages.add(segment.messageId)
+    // O(n) pre-pass: group segments by messageId for O(1) lookups below.
+    const segmentsByMessageId = new Map<string, TimelineSegment[]>()
+    for (const s of props.segments) {
+      let list = segmentsByMessageId.get(s.messageId)
+      if (!list) {
+        list = []
+        segmentsByMessageId.set(s.messageId, list)
+      }
+      list.push(s)
+    }
 
-      const record = resolvedStore.getMessage(segment.messageId)
+    for (const [messageId, segs] of segmentsByMessageId) {
+      const record = resolvedStore.getMessage(messageId)
       if (!record) {
-        for (const s of props.segments) {
-          if (s.messageId === segment.messageId) result[s.id] = s.totalChars
-        }
+        for (const s of segs) result[s.id] = s.totalChars
         continue
       }
 
       const { orderedParts } = buildRecordDisplayData(props.instanceId, record)
       if (!orderedParts?.length) {
-        for (const s of props.segments) {
-          if (s.messageId === segment.messageId) result[s.id] = s.totalChars
-        }
+        for (const s of segs) result[s.id] = s.totalChars
         continue
       }
 
@@ -579,8 +584,7 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
       }
 
       // Assign fresh chars to each segment of this message
-      for (const s of props.segments) {
-        if (s.messageId !== segment.messageId) continue
+      for (const s of segs) {
         const ids = [...(s.partIds ?? []), ...(s.toolPartIds ?? [])]
         let chars = 0
         for (const pid of ids) chars += partChars.get(pid) ?? 0
diff --git a/packages/ui/src/lib/i18n/messages/es/messaging.ts b/packages/ui/src/lib/i18n/messages/es/messaging.ts
index da4be35b..71164f45 100644
--- a/packages/ui/src/lib/i18n/messages/es/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/es/messaging.ts
@@ -91,6 +91,7 @@ export const messagingMessages = {
   "messageSection.bulkDelete.cancelTitle": "Cancelar selección",
   "messageSection.bulkDelete.failedTitle": "Error al eliminar",
   "messageSection.bulkDelete.failedMessage": "No se pudieron eliminar los mensajes seleccionados",
+  "messageSection.bulkDelete.selectionHint": "Ctrl+Click toggle \u00b7 Shift+Click range \u00b7 Esc clear",
   "messageItem.status.queued": "EN COLA",
   "messageItem.status.generating": "Generando...",
   "messageItem.status.sending": "Enviando...",
diff --git a/packages/ui/src/lib/i18n/messages/fr/messaging.ts b/packages/ui/src/lib/i18n/messages/fr/messaging.ts
index 8f4c1af0..af806804 100644
--- a/packages/ui/src/lib/i18n/messages/fr/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/fr/messaging.ts
@@ -91,6 +91,7 @@ export const messagingMessages = {
   "messageSection.bulkDelete.cancelTitle": "Annuler la sélection",
   "messageSection.bulkDelete.failedTitle": "Échec de suppression",
   "messageSection.bulkDelete.failedMessage": "Impossible de supprimer les messages sélectionnés",
+  "messageSection.bulkDelete.selectionHint": "Ctrl+Click toggle \u00b7 Shift+Click range \u00b7 Esc clear",
   "messageItem.status.queued": "EN FILE",
   "messageItem.status.generating": "Génération...",
   "messageItem.status.sending": "Envoi...",
diff --git a/packages/ui/src/lib/i18n/messages/ja/messaging.ts b/packages/ui/src/lib/i18n/messages/ja/messaging.ts
index f07f9789..e7106385 100644
--- a/packages/ui/src/lib/i18n/messages/ja/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/ja/messaging.ts
@@ -91,6 +91,7 @@ export const messagingMessages = {
   "messageSection.bulkDelete.cancelTitle": "選択をキャンセル",
   "messageSection.bulkDelete.failedTitle": "削除に失敗しました",
   "messageSection.bulkDelete.failedMessage": "選択したメッセージの削除に失敗しました",
+  "messageSection.bulkDelete.selectionHint": "Ctrl+Click toggle \u00b7 Shift+Click range \u00b7 Esc clear",
   "messageItem.status.queued": "待機中",
   "messageItem.status.generating": "生成中...",
   "messageItem.status.sending": "送信中...",
diff --git a/packages/ui/src/lib/i18n/messages/ru/messaging.ts b/packages/ui/src/lib/i18n/messages/ru/messaging.ts
index 71430754..f9822382 100644
--- a/packages/ui/src/lib/i18n/messages/ru/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/ru/messaging.ts
@@ -91,6 +91,7 @@ export const messagingMessages = {
   "messageSection.bulkDelete.cancelTitle": "Отменить выбор",
   "messageSection.bulkDelete.failedTitle": "Ошибка удаления",
   "messageSection.bulkDelete.failedMessage": "Не удалось удалить выбранные сообщения",
+  "messageSection.bulkDelete.selectionHint": "Ctrl+Click toggle \u00b7 Shift+Click range \u00b7 Esc clear",
   "messageItem.status.queued": "В ОЧЕРЕДИ",
   "messageItem.status.generating": "Генерация…",
   "messageItem.status.sending": "Отправка…",
diff --git a/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts b/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts
index 71f5c0f8..9b582bf6 100644
--- a/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts
+++ b/packages/ui/src/lib/i18n/messages/zh-Hans/messaging.ts
@@ -91,6 +91,7 @@ export const messagingMessages = {
   "messageSection.bulkDelete.cancelTitle": "取消选择",
   "messageSection.bulkDelete.failedTitle": "删除失败",
   "messageSection.bulkDelete.failedMessage": "无法删除已选择的消息",
+  "messageSection.bulkDelete.selectionHint": "Ctrl+Click toggle \u00b7 Shift+Click range \u00b7 Esc clear",
   "messageItem.status.queued": "排队中",
   "messageItem.status.generating": "正在生成...",
   "messageItem.status.sending": "正在发送...",