diff --git a/src/client/src/chatHistoryCache.test.ts b/src/client/src/chatHistoryCache.test.ts
index 5d57a16..5fd87ff 100644
--- a/src/client/src/chatHistoryCache.test.ts
+++ b/src/client/src/chatHistoryCache.test.ts
@@ -19,12 +19,19 @@ describe("mergeChatHistory", () => {
     expect(mergeChatHistory(existing, incoming)).toEqual(page(0, 4, ["a", "b", "c", "d"]));
   });
 
-  it("uses incoming history when totals shrink", () => {
+  it("uses incoming history when a complete cached history shrinks", () => {
     const incoming = page(0, 2, ["fresh-a", "fresh-b"]);
 
     expect(mergeChatHistory(page(0, 3, ["stale-a", "stale-b", "stale-c"]), incoming)).toEqual(incoming);
   });
 
+  it("keeps adjacent cached history when an older page reports a lower total", () => {
+    const existing = page(100, 200, ["newer-a", "newer-b"]);
+    const incoming = page(98, 150, ["older-a", "older-b"]);
+
+    expect(mergeChatHistory(existing, incoming)).toEqual(page(98, 200, ["older-a", "older-b", "newer-a", "newer-b"]));
+  });
+
   it("uses incoming history instead of creating a gapped page", () => {
     const incoming = page(8, 10, ["i", "j"]);
 
diff --git a/src/client/src/chatHistoryCache.ts b/src/client/src/chatHistoryCache.ts
index 97891f6..a2a4f79 100644
--- a/src/client/src/chatHistoryCache.ts
+++ b/src/client/src/chatHistoryCache.ts
@@ -37,7 +37,7 @@ export function writeChatHistoryCache(sessionId: string, page: RawMessagePage):
 
 export function mergeChatHistory(existing: RawMessagePage | undefined, incoming: RawMessagePage): RawMessagePage {
   if (existing === undefined) return incoming;
-  if (existing.total > incoming.total) return incoming;
+  if (isCompleteReplacement(existing, incoming)) return incoming;
 
   const start = Math.min(existing.start, incoming.start);
   const end = Math.max(existing.start + existing.messages.length, incoming.start + incoming.messages.length);
@@ -46,7 +46,11 @@ export function mergeChatHistory(existing: RawMessagePage | undefined, incoming:
   copyInto(messages, start, incoming);
 
   if (hasSparseEntries(messages)) return incoming;
-  return { start, total: incoming.total, messages };
+  return { start, total: Math.max(existing.total, incoming.total), messages };
+}
+
+function isCompleteReplacement(existing: RawMessagePage, incoming: RawMessagePage): boolean {
+  return existing.total > incoming.total && existing.start === 0 && incoming.start === 0 && incoming.messages.length === incoming.total;
 }
 
 function hasSparseEntries(messages: unknown[]): boolean {
diff --git a/src/client/src/components/ChatView.ts b/src/client/src/components/ChatView.ts
index 2071bc9..a4fc46f 100644
--- a/src/client/src/components/ChatView.ts
+++ b/src/client/src/components/ChatView.ts
@@ -1,5 +1,6 @@
 import { LitElement, html } from "lit";
 import { customElement, property, query, state } from "lit/decorators.js";
+import { repeat } from "lit/directives/repeat.js";
 import { groupChatMessages, summarizeChatGroup } from "../chatGroups";
 import { shouldRequestEarlierMessages } from "../chatHistoryLoading";
 import type { SessionActivity, SessionStatus } from "../api";
@@ -37,7 +38,6 @@ export class ChatView extends LitElement {
   @query(".chat") private chat?: HTMLDivElement;
   @state() private pinnedToBottom = true;
   @state() private openGroupKeys = new Set<string>();
-  @state() private loadedScrollPercent = 100;
   @state() private expandedMetaKey: string | undefined;
   @state() private copiedMessageKey: string | undefined;
   private suppressScrollSave = false;
@@ -77,7 +77,6 @@ export class ChatView extends LitElement {
     if (changed.has("loadingMore") && !this.loadingMore) this.loadMoreRequested = false;
     if (changed.has("hasMore") && !this.hasMore) this.loadMoreRequested = false;
     if (!changed.has("sessionId") && changed.has("messages") && this.pinnedToBottom) this.scrollToBottom();
-    this.updateLoadedScrollPercent();
     if (changed.has("messages") || changed.has("hasMore") || changed.has("loadingMore")) this.requestLoadMoreIfNeeded();
   }
 
@@ -87,9 +86,13 @@ export class ChatView extends LitElement {
         ${this.renderHistoryIndicator()}
         <div class="chat" @scroll=${() => { this.onScroll(); }} @wheel=${(event: WheelEvent) => { this.onWheel(event); }} @touchstart=${(event: TouchEvent) => { this.onTouchStart(event); }} @touchmove=${(event: TouchEvent) => { this.onTouchMove(event); }}>
           ${this.renderHistoryBoundary()}
-          ${groupChatMessages(this.messages, this.messageStart).map((group) => group.kind === "message"
-            ? this.renderMessage(group.message, group.index)
-            : this.renderMessageGroup(group.messages, group.startIndex))}
+          ${repeat(
+            groupChatMessages(this.messages, this.messageStart),
+            (group) => group.kind === "message" ? this.messageAnchorKey(group.index) : this.groupAnchorKey(group.startIndex),
+            (group) => group.kind === "message"
+              ? this.renderMessage(group.message, group.index)
+              : this.renderMessageGroup(group.messages, group.startIndex),
+          )}
           ${this.renderQueuedMessages()}
           ${this.renderSessionActivity()}
         </div>
@@ -174,7 +177,6 @@ export class ChatView extends LitElement {
     return html`
       <div class="history-indicator">
         <div>${fullHistory}</div>
-        <div>loaded scroll: ${String(this.loadedScrollPercent)}% from top</div>
       </div>
     `;
   }
@@ -351,7 +353,6 @@ export class ChatView extends LitElement {
   }
 
   private onScroll() {
-    this.updateLoadedScrollPercent();
     this.requestLoadMoreIfNeeded();
     this.updatePinnedToBottomFromScroll();
     if (!this.suppressScrollSave) this.scheduleScrollPositionSave();
@@ -393,14 +394,6 @@ export class ChatView extends LitElement {
     return chat !== undefined && this.lastClientHeight !== 0 && chat.clientHeight !== this.lastClientHeight;
   }
 
-  private updateLoadedScrollPercent(): void {
-    const chat = this.chat;
-    if (!chat) return;
-    const maxScroll = chat.scrollHeight - chat.clientHeight;
-    const percent = maxScroll <= 0 ? 100 : Math.round((chat.scrollTop / maxScroll) * 100);
-    this.loadedScrollPercent = Math.max(0, Math.min(100, percent));
-  }
-
   private requestLoadMoreIfNeeded(): void {
     requestAnimationFrame(() => {
       const chat = this.chat;
@@ -500,7 +493,6 @@ export class ChatView extends LitElement {
       chat.scrollTop = anchor.scrollTop + (chat.scrollHeight - anchor.scrollHeight);
       this.lastScrollTop = chat.scrollTop;
     });
-    this.updateLoadedScrollPercent();
     this.requestLoadMoreIfNeeded();
   }
 
diff --git a/src/client/src/controllers/sessionController.ts b/src/client/src/controllers/sessionController.ts
index 9773799..6e3bca9 100644
--- a/src/client/src/controllers/sessionController.ts
+++ b/src/client/src/controllers/sessionController.ts
@@ -13,6 +13,8 @@ export class SessionController {
   private readonly socket = new SessionSocket();
   private selectionSeq = 0;
   private catchupStreamSessionId: string | undefined;
+  private pendingTranscriptEvents: SessionUiEvent[] = [];
+  private pendingTranscriptFrame: number | undefined;
 
   constructor(private readonly getState: GetState, private readonly setState: SetState, private readonly updateUrl: UpdateUrl) {}
 
@@ -24,11 +26,13 @@ export class SessionController {
 
   dispose() {
     this.socket.close();
+    this.clearPendingTranscriptEvents();
   }
 
   clearActiveSession() {
     this.socket.close();
     this.catchupStreamSessionId = undefined;
+    this.clearPendingTranscriptEvents();
     this.setState({ selectedSession: undefined, messages: [], messagePageStart: 0, messagePageTotal: 0, isLoadingEarlierMessages: false, isReceivingPartialStream: false, status: undefined, activity: undefined });
   }
 
@@ -48,6 +52,7 @@ export class SessionController {
     const seq = ++this.selectionSeq;
     this.socket.close();
     this.catchupStreamSessionId = undefined;
+    this.clearPendingTranscriptEvents();
     const cached = readChatHistoryCache(session.id);
     this.setState({
       selectedSession: session,
@@ -342,6 +347,12 @@ export class SessionController {
       if (isTranscriptEvent(event)) return;
     }
 
+    if (isHighFrequencyTranscriptEvent(event)) {
+      this.queueTranscriptEvent(event);
+      return;
+    }
+
+    this.flushPendingTranscriptEvents();
     const transcript = applyTranscriptEvent(this.getState().messages, event);
     if (transcript) {
       this.setState({ messages: transcript });
@@ -354,6 +365,31 @@ export class SessionController {
     }
   }
 
+  private queueTranscriptEvent(event: SessionUiEvent): void {
+    this.pendingTranscriptEvents.push(event);
+    if (this.pendingTranscriptFrame !== undefined) return;
+    this.pendingTranscriptFrame = requestAnimationFrame(() => {
+      this.pendingTranscriptFrame = undefined;
+      this.flushPendingTranscriptEvents();
+    });
+  }
+
+  private flushPendingTranscriptEvents(): void {
+    if (this.pendingTranscriptEvents.length === 0) return;
+    const events = this.pendingTranscriptEvents;
+    this.pendingTranscriptEvents = [];
+    let messages = this.getState().messages;
+    for (const event of events) messages = applyTranscriptEvent(messages, event) ?? messages;
+    if (messages !== this.getState().messages) this.setState({ messages });
+  }
+
+  private clearPendingTranscriptEvents(): void {
+    this.pendingTranscriptEvents = [];
+    if (this.pendingTranscriptFrame === undefined) return;
+    cancelAnimationFrame(this.pendingTranscriptFrame);
+    this.pendingTranscriptFrame = undefined;
+  }
+
   private finishStreamCatchup(sessionId: string) {
     if (this.catchupStreamSessionId !== sessionId) return;
     this.catchupStreamSessionId = undefined;
@@ -377,3 +413,7 @@ function isTranscriptEvent(event: SessionUiEvent): boolean {
   return ["message.append", "assistant.delta", "assistant.thinking.delta", "tool.start", "tool.end", "shell.start", "shell.chunk", "shell.end", "command.output", "session.error"].includes(event.type);
 }
 
+function isHighFrequencyTranscriptEvent(event: SessionUiEvent): boolean {
+  return event.type === "assistant.delta" || event.type === "assistant.thinking.delta" || event.type === "shell.chunk";
+}
+
diff --git a/src/client/src/formatting/markdown.ts b/src/client/src/formatting/markdown.ts
index 8eaaa52..99c5eb7 100644
--- a/src/client/src/formatting/markdown.ts
+++ b/src/client/src/formatting/markdown.ts
@@ -3,9 +3,20 @@ import { marked } from "marked";
 const renderer = new marked.Renderer();
 renderer.html = ({ text }) => escapeHtml(text);
 
+const MAX_MARKDOWN_CACHE_ENTRIES = 300;
+const markdownHtmlCache = new Map<string, string>();
+
 export function toSafeMarkdownHtml(text: string): string {
+  const cached = markdownHtmlCache.get(text);
+  if (cached !== undefined) return cached;
   const html = marked.parse(text, { async: false, breaks: true, gfm: true, renderer });
-  return sanitizeHtml(html);
+  const safeHtml = sanitizeHtml(html);
+  markdownHtmlCache.set(text, safeHtml);
+  if (markdownHtmlCache.size > MAX_MARKDOWN_CACHE_ENTRIES) {
+    const oldest = markdownHtmlCache.keys().next().value;
+    if (oldest !== undefined) markdownHtmlCache.delete(oldest);
+  }
+  return safeHtml;
 }
 
 function escapeHtml(text: string): string {