diff --git a/apps/backend/src/middleware/http-cache.test.ts b/apps/backend/src/middleware/http-cache.test.ts
index 0477e622..888a3eaa 100644
--- a/apps/backend/src/middleware/http-cache.test.ts
+++ b/apps/backend/src/middleware/http-cache.test.ts
@@ -51,3 +51,26 @@ test('dev-mock/status 不能加 HTTP 缓存（它是可切换状态，缓存会
     'dev-mock/status 缓存后，POST 切换开关再读会拿到旧值，界面看起来没生效',
   )
 })
+
+test('实时广播必须一次序列化、多路复用（不能在每个订阅者循环里各 stringify 一次）', async () => {
+  const src = await readFile(
+    new URL('../services/realtime/realtime-event-service.ts', import.meta.url),
+    'utf8',
+  )
+  assert.ok(src.includes('function buildEventFrame('), '缺少预构造 SSE 帧的函数')
+  const broadcastBody = src.slice(
+    src.indexOf('function broadcast('),
+    src.indexOf('/** 预先构造一整帧 SSE 报文'),
+  )
+  assert.ok(broadcastBody.includes('const frame = buildEventFrame('), '广播必须先构造一次帧')
+  const broadcastCode = broadcastBody
+    .split('\n')
+    .filter((line) => !line.trim().startsWith('//'))
+    .join('\n')
+  assert.equal(
+    broadcastCode.includes('JSON.stringify'),
+    false,
+    '广播代码里不能出现 JSON.stringify：那意味着每个订阅者各序列化一次（生产实测 67 条/秒 × 客户端数）',
+  )
+  assert.ok(broadcastBody.includes('writeEventFrame(subscriber, frame)'), '循环里只应写入同一帧')
+})
diff --git a/apps/backend/src/services/realtime/realtime-event-service.ts b/apps/backend/src/services/realtime/realtime-event-service.ts
index af678e54..4a36a7a1 100644
--- a/apps/backend/src/services/realtime/realtime-event-service.ts
+++ b/apps/backend/src/services/realtime/realtime-event-service.ts
@@ -471,24 +471,37 @@ function isEventVisibleToSubscriber(event: StoredRealtimeEvent, subscriber: Real
 }
 
 function broadcast(subscribers: Iterable<RealtimeSubscriber>, event: RealtimeEvent) {
+  // 同一份载荷要发给 N 个订阅者，序列化必须只做一次。
+  // 生产实测单个后台客户端就能收到约 67 条/秒、每条约 2.7KB 的事件，
+  // 原实现在订阅者循环里各写一次 JSON.stringify + 模板串，客户端越多越吃亏。
+  const frame = buildEventFrame('realtime', event)
   for (const subscriber of subscribers) {
-    writeEvent(subscriber, 'realtime', event)
+    writeEventFrame(subscriber, frame)
   }
 }
 
+/** 预先构造一整帧 SSE 报文（含 `id:` 行），供广播复用。 */
+function buildEventFrame(name: string, payload: unknown): string {
+  const eventId =
+    name === 'realtime' && payload && typeof payload === 'object' && 'eventId' in payload
+      ? Number((payload as { eventId?: unknown }).eventId || 0)
+      : 0
+  const idLine = eventId > 0 ? `id: ${eventId}\n` : ''
+  return `${idLine}event: ${name}\ndata: ${JSON.stringify(payload)}\n\n`
+}
+
 function writeEvent(subscriber: RealtimeSubscriber, name: string, payload: unknown) {
+  writeEventFrame(subscriber, buildEventFrame(name, payload))
+}
+
+function writeEventFrame(subscriber: RealtimeSubscriber, frame: string) {
   if (subscriber.response.writableEnded || subscriber.response.destroyed) {
     removeSubscriber(subscriber)
     return
   }
 
   try {
-    const eventId =
-      name === 'realtime' && payload && typeof payload === 'object' && 'eventId' in payload
-        ? Number((payload as { eventId?: unknown }).eventId || 0)
-        : 0
-    const idLine = eventId > 0 ? `id: ${eventId}\n` : ''
-    subscriber.response.write(`${idLine}event: ${name}\ndata: ${JSON.stringify(payload)}\n\n`)
+    subscriber.response.write(frame)
   } catch {
     removeSubscriber(subscriber)
   }