diff --git a/docs/roadmap/product-performance-experience-improvement-plan.md b/docs/roadmap/product-performance-experience-improvement-plan.md
index 8c008b40..759483d5 100644
--- a/docs/roadmap/product-performance-experience-improvement-plan.md
+++ b/docs/roadmap/product-performance-experience-improvement-plan.md
@@ -605,6 +605,38 @@ Renderer 主线程降速 4 倍（Main 不降速，每次 reload 后重新应用
 低成本热点：Composer 高度改为只在需要时测量、`t()` 结果或 i18next 语言码规范化做缓存、
 滚动跟随按帧合并且只在贴底时执行。
 
+#### 2026-10-02 第二步：三处低成本热点
+
+- **Composer 高度（`App.tsx` `resizeComposerTextarea`）：** 记住上次的值与高度。值未变不测量；
+  在原文本末尾追加时不再先设 `height: auto`（只会变高），已到最大高度直接跳过测量；删除、
+  替换、清空仍按原方式重置并测量。
+- **i18next 语言码（`i18n/index.ts`）：** i18next 25 每次 `t()` 都经
+  `toResolveHierarchy`→`formatLanguageCode` 调用 `Intl.getCanonicalLocales`。初始化后对
+  `languageUtils.formatLanguageCode` 加按输入缓存（结果只取决于代码与静态初始化选项，上限
+  64 项），翻译结果不变。
+- **滚动跟随（`ChatHistoryPane.tsx`）：** 激活面板时仍同步滚到底；之后内容更新时改为每帧最多
+  一次 `requestAnimationFrame` 中读取 `scrollHeight` 并 `scrollTo`，回调里再次确认仍贴底；
+  卸载时取消。读者离开底部时不滚动，恢复阅读位置逻辑不变。
+- **测试：** 新增“流式三次更新只滚动一帧”（旧实现 3 次，失败）、Composer 追加与删除的测量
+  次数、`t()` 不再调用 `Intl.getCanonicalLocales`。全量测试 5,621 通过，2 失败与本改动
+  无关（真实 OpenCode 集成测试因免费额度返回 403；Supervisor 超时设置用例此前已失败）。
+- **`npm run perf:app` A/B（同机，改动前为临时 worktree 中的 `983cdd4` 构建，交替运行）：**
+
+| 场景（4× 降速，3 轮） | 改动前 按键/点击 p50 / p95 | 改动后 |
+| --- | --- | --- |
+| 输入 120 字（新 profile） | 28–80 / 33–361 ms | 19–36 / 24–57 ms |
+| 输入 120 字（有 300 个会话） | 265–2,372 / 1,486–4,266 ms | 27–161 / 32–346 ms |
+| 首次切换会话 | 442–761 / 1,063–1,384 ms | 360–398 / 820–1,334 ms |
+| 再次切换（缓存命中） | 472–749 / 501–1,143 ms | 390–420 / 456–951 ms |
+| 流式 40 KB：Long Tasks 总时长 | 15.9–20.6 s | 15.1–17.4 s |
+| 流式期间输入 | 432–6,778 / 1,146–9,634 ms | 96–466 / 297–1,567 ms |
+
+  不降速时（2 轮）两版各场景持平，输入 p95 均约 9 ms，流式 1 个 Long Task。4× 降速下
+  方差很大（同版本轮间可差数倍），应看量级：有历史时输入从秒级回到百毫秒内，切换略有改善，
+  流式本身仍是每帧卡顿——剩余成本是 App 级重渲染与 Markdown/React 提交，需 `PERF-13`。
+- **剩余：** 流式 delta 按帧合并后以 transition 提交、大面板 memo 仍待实施；之后进入
+  `PERF-13`。
+
 ### PERF-13 Renderer 领域 Store 迁移
 
 - **优先级 / 状态：** P1 / 待开始
diff --git a/src/renderer/src/App.test.tsx b/src/renderer/src/App.test.tsx
index 3371cccd..6a19c4de 100644
--- a/src/renderer/src/App.test.tsx
+++ b/src/renderer/src/App.test.tsx
@@ -3660,7 +3660,7 @@ describe("App", () => {
   it("measures composer height once per input and resizes after sending", async () => {
     render(<App />);
     const input = await screen.findByLabelText("向 GoodBuddy 提问");
-    const measureHeight = vi.fn(() =>
+    const measureHeight = vi.fn((): number =>
       (input as HTMLTextAreaElement).value ? 300 : 0,
     );
     Object.defineProperty(input, "scrollHeight", {
@@ -3672,6 +3672,21 @@ describe("App", () => {
     expect(measureHeight).toHaveBeenCalledTimes(1);
     expect(input).toHaveStyle({ height: "220px" });
 
+    // Appending at the maximum height cannot change the size.
+    measureHeight.mockClear();
+    fireEvent.input(input, { target: { value: "First line\nSecond line!" } });
+    expect(measureHeight).not.toHaveBeenCalled();
+    expect(input).toHaveStyle({ height: "220px" });
+
+    // Deleting text may shrink the box, so it is measured again.
+    measureHeight.mockImplementation(() => 100);
+    fireEvent.input(input, { target: { value: "First" } });
+    expect(measureHeight).toHaveBeenCalledTimes(1);
+    expect(input).toHaveStyle({ height: "100px" });
+
+    measureHeight.mockImplementation(() =>
+      (input as HTMLTextAreaElement).value ? 300 : 0,
+    );
     measureHeight.mockClear();
     fireEvent.click(screen.getByLabelText("发送"));
     await waitFor(() => expect(input).toHaveValue(""));
@@ -4477,6 +4492,51 @@ describe("App", () => {
     expect(screen.getByRole("button", { name: "到底部" })).toBeInTheDocument();
   });
 
+  it("follows streaming output at the bottom once per frame", async () => {
+    render(<App />);
+    fireEvent.change(await screen.findByLabelText("向 GoodBuddy 提问"), {
+      target: { value: "生成流式回复" },
+    });
+    fireEvent.click(screen.getByLabelText("发送"));
+    await waitFor(() => expect(run).toHaveBeenCalledOnce());
+    const request = run.mock.calls[0]?.[0];
+    if (!request) {
+      throw new Error("Missing request");
+    }
+    await act(
+      () =>
+        new Promise<void>((resolve) => requestAnimationFrame(() => resolve())),
+    );
+    const chat = document.querySelector<HTMLElement>(".chat");
+    if (!chat) {
+      throw new Error("Missing chat scroll container");
+    }
+    let scrollHeight = 1_200;
+    Object.defineProperties(chat, {
+      clientHeight: { configurable: true, value: 400 },
+      scrollHeight: { configurable: true, get: () => scrollHeight },
+      scrollTop: { configurable: true, writable: true, value: 800 },
+    });
+    const scrollTo = vi.fn();
+    chat.scrollTo = scrollTo;
+
+    for (const delta of ["第一段", "第二段", "第三段"]) {
+      scrollHeight += 100;
+      act(() => {
+        agentListener?.({ requestId: request.requestId, type: "text", delta });
+      });
+    }
+    expect(await screen.findByText(/第三段/u)).toBeInTheDocument();
+    await act(
+      () =>
+        new Promise<void>((resolve) => requestAnimationFrame(() => resolve())),
+    );
+
+    expect(scrollTo).toHaveBeenCalled();
+    expect(scrollTo.mock.calls.length).toBeLessThanOrEqual(2);
+    expect(scrollTo).toHaveBeenLastCalledWith({ top: 1_500, behavior: "auto" });
+  });
+
   it("restores the reader position after activity continues on another page", async () => {
     const { container } = render(<App />);
     fireEvent.change(await screen.findByLabelText("向 GoodBuddy 提问"), {
diff --git a/src/renderer/src/App.tsx b/src/renderer/src/App.tsx
index 6bfd5f54..2277fac0 100644
--- a/src/renderer/src/App.tsx
+++ b/src/renderer/src/App.tsx
@@ -1354,15 +1354,51 @@ function formatAttachmentSize(size: number): string {
 const composerTextareaMinHeight = 72;
 const composerTextareaMaxHeight = 220;
 
+const composerTextareaSizes = new WeakMap<
+  HTMLTextAreaElement,
+  { height: number; value: string }
+>();
+
+function clampComposerTextareaHeight(height: number): number {
+  return Math.max(
+    composerTextareaMinHeight,
+    Math.min(height, composerTextareaMaxHeight),
+  );
+}
+
+// Sizing reads scrollHeight, which forces a synchronous layout of the whole
+// window. Skip the read when the result is known, and avoid the extra
+// "auto" reset layout when text was only appended (it can only grow).
 function resizeComposerTextarea(textarea: HTMLTextAreaElement | null): void {
   if (!textarea) {
     return;
   }
-  textarea.style.height = "auto";
-  textarea.style.height = `${Math.max(
-    composerTextareaMinHeight,
-    Math.min(textarea.scrollHeight, composerTextareaMaxHeight),
-  )}px`;
+  const value = textarea.value;
+  const previous = composerTextareaSizes.get(textarea);
+  const previousHeightApplied =
+    previous !== undefined &&
+    textarea.style.height === `${previous.height}px`;
+  let height: number;
+  if (previousHeightApplied && value === previous.value) {
+    height = previous.height;
+  } else if (
+    previousHeightApplied &&
+    previous.value !== "" &&
+    value.startsWith(previous.value)
+  ) {
+    height =
+      previous.height >= composerTextareaMaxHeight
+        ? composerTextareaMaxHeight
+        : clampComposerTextareaHeight(textarea.scrollHeight);
+  } else {
+    textarea.style.height = "auto";
+    height = clampComposerTextareaHeight(textarea.scrollHeight);
+  }
+  const nextHeight = `${height}px`;
+  if (textarea.style.height !== nextHeight) {
+    textarea.style.height = nextHeight;
+  }
+  composerTextareaSizes.set(textarea, { height, value });
 }
 
 const imageDataUrlPattern = /^data:image\/(png|jpeg|webp);base64,/u;
diff --git a/src/renderer/src/ChatHistoryPane.tsx b/src/renderer/src/ChatHistoryPane.tsx
index 11bfe327..882c0a7e 100644
--- a/src/renderer/src/ChatHistoryPane.tsx
+++ b/src/renderer/src/ChatHistoryPane.tsx
@@ -109,6 +109,7 @@ export const ChatHistoryPane = memo(function ChatHistoryPane({
   const latestScrollSnapshotRef = useRef(scrollSnapshot);
   const restorePendingRef = useRef(true);
   const wasActiveRef = useRef(false);
+  const followFrameRef = useRef<number | undefined>(undefined);
   const prependScrollPositionRef = useRef<
     | {
         scrollHeight: number;
@@ -260,14 +261,42 @@ export const ChatHistoryPane = memo(function ChatHistoryPane({
         return;
       }
     }
-    if (pinnedToBottomRef.current) {
+    if (!pinnedToBottomRef.current) {
+      return;
+    }
+    if (activated) {
       scrollContainer.scrollTo({
         top: scrollContainer.scrollHeight,
         behavior: "auto",
       });
+      return;
     }
+    // Streaming updates commit many times per frame. Reading scrollHeight here
+    // would force a synchronous layout per commit, so follow the bottom once
+    // per frame instead; rAF still runs before the frame is painted.
+    if (followFrameRef.current !== undefined) {
+      return;
+    }
+    followFrameRef.current = requestAnimationFrame(() => {
+      followFrameRef.current = undefined;
+      const container = scrollRef.current;
+      if (!container || !pinnedToBottomRef.current) {
+        return;
+      }
+      container.scrollTo({ top: container.scrollHeight, behavior: "auto" });
+    });
   }, [active, conversation.messages, scrollSnapshot, visibleMessageCount]);
 
+  useEffect(
+    () => () => {
+      if (followFrameRef.current !== undefined) {
+        cancelAnimationFrame(followFrameRef.current);
+        followFrameRef.current = undefined;
+      }
+    },
+    [],
+  );
+
   useLayoutEffect(() => {
     const previous = prependScrollPositionRef.current;
     if (!previous) {
diff --git a/src/renderer/src/i18n/i18n.test.ts b/src/renderer/src/i18n/i18n.test.ts
index d24daf59..3386228b 100644
--- a/src/renderer/src/i18n/i18n.test.ts
+++ b/src/renderer/src/i18n/i18n.test.ts
@@ -1,4 +1,4 @@
-import { afterEach, describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import i18n, {
   changeUiLocale,
   i18nResources
@@ -41,4 +41,20 @@ describe('renderer i18n resources', () => {
     await changeUiLocale('zh-CN')
     expect(i18n.t('navigation.chat')).toBe('对话')
   })
+
+  it('canonicalizes language codes once instead of on every translation', async () => {
+    const getCanonicalLocales = vi.spyOn(Intl, 'getCanonicalLocales')
+    try {
+      await changeUiLocale('en-US')
+      expect(i18n.t('navigation.chat')).toBe('Chat')
+      getCanonicalLocales.mockClear()
+      for (let index = 0; index < 50; index += 1) {
+        i18n.t('navigation.chat')
+      }
+      expect(getCanonicalLocales).not.toHaveBeenCalled()
+      expect(i18n.services.languageUtils.formatLanguageCode('zh-cn')).toBe('zh-CN')
+    } finally {
+      getCanonicalLocales.mockRestore()
+    }
+  })
 })
diff --git a/src/renderer/src/i18n/index.ts b/src/renderer/src/i18n/index.ts
index 608ffa1f..f48488a4 100644
--- a/src/renderer/src/i18n/index.ts
+++ b/src/renderer/src/i18n/index.ts
@@ -68,6 +68,37 @@ void i18n
     returnNull: false
   })
 
+memoizeLanguageCodeFormatting(i18n)
+
+/**
+ * i18next canonicalizes the current language with `Intl.getCanonicalLocales`
+ * on every `t()` call. The result depends only on the code and the static init
+ * options, so cache it; large lists call `t()` hundreds of times per render.
+ */
+export function memoizeLanguageCodeFormatting(instance: typeof i18n): void {
+  const languageUtils = instance.services?.languageUtils as
+    | { formatLanguageCode?: (code: string) => string }
+    | undefined
+  const format = languageUtils?.formatLanguageCode
+  if (!languageUtils || typeof format !== 'function') {
+    return
+  }
+  const cache = new Map<string, string>()
+  languageUtils.formatLanguageCode = (code: string): string => {
+    if (typeof code !== 'string') {
+      return format.call(languageUtils, code)
+    }
+    let formatted = cache.get(code)
+    if (formatted === undefined) {
+      formatted = format.call(languageUtils, code)
+      if (cache.size < 64) {
+        cache.set(code, formatted)
+      }
+    }
+    return formatted
+  }
+}
+
 export async function changeUiLocale(locale: UiLocale): Promise<void> {
   await i18n.changeLanguage(locale)
   document.documentElement.lang = locale