diff --git a/app/api/v1/plugins.py b/app/api/v1/plugins.py
index 46f5c1f..f67ffed 100644
--- a/app/api/v1/plugins.py
+++ b/app/api/v1/plugins.py
@@ -693,6 +693,72 @@ class PreviewRequest(BaseModel):
     preset_data: Optional[str] = None  # base64 bytes .vstpreset (project nhúng)
 
 
+# M24: thao tac GUI native (hide/open/close editor in-process fxrt) co work
+# blocking (capture_editor_preset poll file time.sleep 0.05 x3s,
+# wait_chain_ready sleep-poll, terminate proc.wait(5)). Truoc M24 cac sleep nay
+# chay DONG BO trong async endpoint -> block event loop uvicorn -> pump audio
+# `_pump_output` (cung loop) stall 60-200ms -> worklet queue can -> underrun ->
+# am lag roi moi play lai moi lan switch GUI luc dang play (log fxrt_pump.log:
+# pump_gap ngay sau GUI op). Fix: chay blocking work trong thread
+# (asyncio.to_thread) duoi lock per-session de GIU thu tu hide->open nhu cu.
+_FXGUI_OP_LOCKS = {}           # session name -> asyncio.Lock
+_FXGUI_OP_LOCKS_GUARD = threading.Lock()
+
+def _fxgui_op_lock(session_name):
+    with _FXGUI_OP_LOCKS_GUARD:
+        lock = _FXGUI_OP_LOCKS.get(session_name)
+        if lock is None:
+            lock = asyncio.Lock()
+            _FXGUI_OP_LOCKS[session_name] = lock
+        return lock
+
+def _fxgui_open_native_blocking(sess, plugin_path):
+    """M24 worker thread (asyncio.to_thread) cho nhanh mo GUI native: cho chain
+    ready tren bridge, terminate fallback GUI cu, gui ed.open + retry 700ms.
+    Tra dict ket qua native; None neu plugin khong phai slot chain (roi xuong
+    fallback); raise HTTPException 409 khi session chet giua chung."""
+    _slot = sess.chain_slot_for_path(plugin_path)
+    if _slot is None:
+        return None
+    _ready = sess.wait_chain_ready(timeout=15)
+    logger.info("[fxgui] open native path=%s shm=%s slot=%s ready=%s",
+                plugin_path, sess.name, _slot, _ready)
+    if not _ready:
+        # M13: chua READY (proc respawn / chain reload cham) — cho them
+        # 5s; van chet thi ed.open chi vao ctrl ring (C++ KHONG reset
+        # ring khi respawn → proc moi doc duoc) nen van gui duoc.
+        sess.wait_chain_ready(timeout=5)
+        if not sess.alive():
+            logger.warning("[fxgui] open native bo — session chet path=%s", plugin_path)
+            raise HTTPException(status_code=409,
+                                detail="Session FX realtime đã đóng — bật lại FX PWR rồi thử mở GUI.")
+    _killed = _terminate_fx_gui_for_path(plugin_path)
+    _already = fx_realtime.editor_find(plugin_path)
+    _same = _already is not None and _already[0] == sess.name
+    _sent = sess.send_editor_cmd("open", slot=_slot)
+    logger.info("[fxgui] open sent=%s killed_fallback=%s already=%s same=%s proc_pid=%s state=%s",
+                _sent, _killed, bool(_already), _same,
+                sess.proc.pid if getattr(sess, "proc", None) else None,
+                getattr(getattr(sess, "h", None), "state", "?"))
+    fx_realtime.editor_register(plugin_path, sess.name, _slot)
+    # M13 workaround click-1: gui doi khi khong hien window ngay (owner
+    # gan sau show → Win32 an window). Gửi lai ed.open sau 700ms = ban
+    # sao click-2 — C++ log "ALREADY open — re-show" thi re-show chinh
+    # la dieu can. ponytail: bo block nay khi log A (prefix p/t) xac
+    # nhan root cause that (show truoc owner).
+    def _retry_open(delay):
+        _time.sleep(delay)
+        try:
+            if sess.alive():
+                sess.send_editor_cmd("open", slot=_slot)
+                logger.info("[fxgui] retry ed.open path=%s slot=%s", plugin_path, _slot)
+        except Exception:
+            pass
+    threading.Thread(target=_retry_open, args=(0.7,), daemon=True).start()
+    return {"success": True, "started": not _same,
+            "already_running": bool(_same), "native": True,
+            "embed_url": None, "session_id": sess.name, "slot": _slot}
+
 @router.post("/fx-gui")
 async def open_fx_gui(req: FxGuiRequest, current_user: dict = Depends(get_current_user)):
     """Mở native GUI của VST FX (mastering chain / FX rack) qua bridge.
@@ -726,62 +792,25 @@ async def open_fx_gui(req: FxGuiRequest, current_user: dict = Depends(get_curren
     # JS set_chain async / watchdog respawn) -> chờ tối đa 15s cho slot xuất hiện;
     # session chết giữa chừng -> bỏ (fallback embed bị chặn bên dưới cho master).
     if req.master and _sess is not None and _sess.alive() and _sess.chain_slot_for_path(plugin_path) is None:
+        # M24: poll async (khong time.sleep dong bo — block event loop lam pump
+        # audio stall). Y nghia giu nguyen: cho toi da 15s slot xuat hien.
         for _ in range(150):
             if not _sess.alive():
                 break
             if _sess.chain_slot_for_path(plugin_path) is not None:
                 break
-            _time.sleep(0.1)
+            await asyncio.sleep(0.1)
     if _sess is not None and _sess.alive():
-        _slot = _sess.chain_slot_for_path(plugin_path)
-        if _slot is not None:
-            # Bug 2-GUI: fallback bridge fx-gui (embed/float) còn sống cho cùng
-            # plugin — instance VST THỨ 2 + window riêng; mở native phải loại nó
-            # trước, nếu không 1 plugin hiện 2 GUI (1 floating native + 1 embed)
-            # và floating không tắt được qua UI. Terminate proc cũ → frontend
-            # embed panel của nó onDead → tự đóng.
-            # M8: set_chain (juce bridge) reload async (watchdog respawn) —
-            # cho chain moi ap tren bridge truoc khi ed.open, khong thi C++
-            # bao "slot ngoai chain (M) — bo qua". Khong co reload dang chay
-            # -> tra ngay (khong them latency mo GUI).
-            _ready = _sess.wait_chain_ready(timeout=15)
-            logger.info("[fxgui] open native path=%s shm=%s slot=%s ready=%s",
-                        plugin_path, _sess.name, _slot, _ready)
-            if not _ready:
-                # M13: chua READY (proc respawn / chain reload cham) — cho them
-                # 5s; van chet thi ed.open chi vao ctrl ring (C++ KHONG reset
-                # ring khi respawn → proc moi doc duoc) nen van gui duoc.
-                _sess.wait_chain_ready(timeout=5)
-                if not _sess.alive():
-                    logger.warning("[fxgui] open native bo — session chet path=%s", plugin_path)
-                    raise HTTPException(status_code=409,
-                                        detail="Session FX realtime đã đóng — bật lại FX PWR rồi thử mở GUI.")
-            _killed = _terminate_fx_gui_for_path(plugin_path)
-            _already = fx_realtime.editor_find(plugin_path)
-            _same = _already is not None and _already[0] == _sess.name
-            _sent = _sess.send_editor_cmd("open", slot=_slot)
-            logger.info("[fxgui] open sent=%s killed_fallback=%s already=%s same=%s proc_pid=%s state=%s",
-                        _sent, _killed, bool(_already), _same,
-                        _sess.proc.pid if getattr(_sess, "proc", None) else None,
-                        getattr(getattr(_sess, "h", None), "state", "?"))
-            fx_realtime.editor_register(plugin_path, _sess.name, _slot)
-            # M13 workaround click-1: gui doi khi khong hien window ngay (owner
-            # gan sau show → Win32 an window). Gửi lai ed.open sau 700ms = ban
-            # sao click-2 — C++ log "ALREADY open — re-show" thi re-show chinh
-            # la dieu can. ponytail: bo block nay khi log A (prefix p/t) xac
-            # nhan root cause that (show truoc owner).
-            def _retry_open(delay):
-                _time.sleep(delay)
-                try:
-                    if _sess.alive():
-                        _sess.send_editor_cmd("open", slot=_slot)
-                        logger.info("[fxgui] retry ed.open path=%s slot=%s", plugin_path, _slot)
-                except Exception:
-                    pass
-            threading.Thread(target=_retry_open, args=(0.7,), daemon=True).start()
-            return {"success": True, "started": not _same,
-                    "already_running": bool(_same), "native": True,
-                    "embed_url": None, "session_id": _sess.name, "slot": _slot}
+        # M24: toan bo nhanh native (wait_chain_ready sleep-poll, terminate
+        # fallback proc.wait, ed.open) chay trong worker thread duoi lock per
+        # session — event loop khong block -> pump audio khong stall khi switch
+        # GUI luc dang play. None khi plugin chua phai slot -> roi xuong
+        # fallback ben duoi nhu cu.
+        async with _fxgui_op_lock(_sess.name):
+            _native = await asyncio.to_thread(
+                _fxgui_open_native_blocking, _sess, plugin_path)
+        if _native is not None:
+            return _native
     # Tránh mở 2 GUI cùng plugin: editor NATIVE (instance DSP trong session
     # fxrt) của plugin này đang mở ở session SỐNG → KHÔNG spawn instance 2 /
     # bridge fx-gui nữa (nguồn CPU 100% + 2 GUI); trả native để frontend hiện
@@ -904,8 +933,12 @@ async def close_fx_gui(req: FxGuiCloseRequest, current_user: dict = Depends(get_
             # M5: capture state editor TRƯỚC khi đóng (preset user chỉnh trong
             # GUI → lưu về project như WM_FXGUI_CAPTURE cũ; preset_b64 dùng để
             # mở lại GUI / nạp chain với đúng cài đặt). Không lỗi nếu rỗng.
-            _preset = _sess.capture_editor_preset(_slot)
-            _sess.send_editor_cmd("close", slot=_slot)
+            # M24: capture (poll file sleep toi 3s) trong thread duoi lock —
+            # khong block event loop/pump audio.
+            async with _fxgui_op_lock(_sess.name):
+                _preset = await asyncio.to_thread(
+                    _sess.capture_editor_preset, _slot)
+                _sess.send_editor_cmd("close", slot=_slot)
             return {"success": True, "closed": True, "native": True,
                     "preset_b64": _preset}
         # session đã chết — rơi xuống spawn fallback (đóng bridge fx-gui cũ nếu có)
@@ -958,8 +991,12 @@ async def hide_fx_gui(req: FxGuiNativeRequest,
     # M13: capture preset TRUOC khi an — dong mastering panel / save project
     # giua luc chinh GUI native khong mat preset moi (truoc day chi capture o
     # /fx-gui/close; hide chi an -> chain luu preset cu/rong sau khi reload).
-    _preset = sess.capture_editor_preset(slot)
-    sess.send_editor_cmd("hide", slot=slot)
+    # M24: capture (poll file sleep toi 3s) trong thread duoi lock — day la
+    # blocker chinh moi lan switch GUI (JS fire-and-forget hide truoc open);
+    # truoc day sleep dong bo trong async endpoint -> pump audio stall -> lag.
+    async with _fxgui_op_lock(sess.name):
+        _preset = await asyncio.to_thread(sess.capture_editor_preset, slot)
+        sess.send_editor_cmd("hide", slot=slot)
     return {"success": True, "applied": True, "native": True,
             "preset_b64": _preset}