diff --git a/controller/cuda_grid_controller/__main__.py b/controller/cuda_grid_controller/__main__.py
index a80867e..5e1ca72 100644
--- a/controller/cuda_grid_controller/__main__.py
+++ b/controller/cuda_grid_controller/__main__.py
@@ -21,6 +21,7 @@ from .frigate_bridge import FrigateBridge, FrigateBridgeCfg
 from .http_api import create_app
 from .mqtt_loop import MqttLoop
 from .snapshot_history import SnapshotHistory
+from .snapshot_keeper import SnapshotKeeper
 from .state import ControllerState
 from .watchdog import StreamWatchdog, WatchdogCfg
 
@@ -111,6 +112,10 @@ async def _run(cfg: Config) -> None:
     # Snapshot history (Phase 6+) — periodic capture per instance
     snapshot_hist = SnapshotHistory(cfg)
 
+    # Snapshot keeper — persistent ffmpeg per instance dumping latest frame
+    # к PNG (для low-latency /snapshot UI polling).
+    snapshot_keeper = SnapshotKeeper(cfg, Path("/tmp/snapshot-keeper"))
+
     # Stream watchdog (Phase 1 resilience, issue #3) — monitor mediamtx paths
     watchdog: StreamWatchdog | None = None
     if cfg.watchdog:
@@ -125,6 +130,7 @@ async def _run(cfg: Config) -> None:
     # HTTP REST
     app = create_app(cfg, state, dispatcher,
                      snapshot_history=snapshot_hist,
+                     snapshot_keeper=snapshot_keeper,
                      frigate_bridge=frigate_bridge)
     server = uvicorn.Server(
         uvicorn.Config(
@@ -149,6 +155,7 @@ async def _run(cfg: Config) -> None:
     if browser_renderer:
         await browser_renderer.start()
     await snapshot_hist.start()
+    await snapshot_keeper.start()
     if watchdog:
         watchdog._publish_event = mqtt.publish_event
         await watchdog.start()
@@ -178,6 +185,7 @@ async def _run(cfg: Config) -> None:
         if browser_renderer:
             await browser_renderer.stop()
         await snapshot_hist.stop()
+        await snapshot_keeper.stop()
         if watchdog:
             await watchdog.stop()
         await dispatcher.close()
diff --git a/controller/cuda_grid_controller/http_api.py b/controller/cuda_grid_controller/http_api.py
index 5b04c84..0f3413a 100644
--- a/controller/cuda_grid_controller/http_api.py
+++ b/controller/cuda_grid_controller/http_api.py
@@ -41,6 +41,7 @@ class AudioOutputReq(BaseModel):
 def create_app(
     cfg: Config, state: ControllerState, dispatcher: CommandDispatcher,
     snapshot_history: SnapshotHistory | None = None,
+    snapshot_keeper=None,
     frigate_bridge=None,
 ) -> FastAPI:
     app = FastAPI(
@@ -221,11 +222,23 @@ def create_app(
         },
     )
     async def snapshot(instance: str) -> Response:
-        """Capture single frame от output_rtsp_url. Returns PNG bytes (image/png)."""
+        """Capture single frame от output_rtsp_url. Returns PNG bytes (image/png).
+
+        Fast path: persistent ffmpeg-keeper subprocess дампит latest frame в
+        файл каждые 500 ms — endpoint просто читает (latency ~10 ms). Cold path:
+        fork ffmpeg на каждый request (~5 sec) — fallback если keeper ещё не
+        создал файл (первые 1-2 sec после controller start) или у инстанса
+        нет output_rtsp_url.
+        """
         inst = _check_instance(instance)
         if not inst.output_rtsp_url:
             raise HTTPException(400, f"instance '{instance}' has no output_rtsp_url configured")
 
+        if snapshot_keeper:
+            path = snapshot_keeper.path(instance)
+            if path:
+                return FileResponse(path, media_type="image/png")
+
         proc = await asyncio.create_subprocess_exec(
             "ffmpeg", "-hide_banner", "-loglevel", "error",
             "-rtsp_transport", "tcp",
diff --git a/controller/cuda_grid_controller/snapshot_keeper.py b/controller/cuda_grid_controller/snapshot_keeper.py
new file mode 100644
index 0000000..951002d
--- /dev/null
+++ b/controller/cuda_grid_controller/snapshot_keeper.py
@@ -0,0 +1,109 @@
+"""Persistent ffmpeg subprocess для low-latency /snapshot endpoint.
+
+Cold ffmpeg start + RTSP negotiate + keyframe wait занимает ~5 sec — это
+неприемлемо для UI editor где preview обновляется каждые ~700 ms. Заменяем
+"per-request ffmpeg" на single long-running ffmpeg который непрерывно читает
+RTSP и dump'ит latest frame в file (`fps=2 -update 1`). Endpoint просто
+serves этот file — latency ~50 ms (disk read).
+
+Auto-restart при exit (RTSP source перезапустился, network glitch и т.д.).
+"""
+
+from __future__ import annotations
+
+import asyncio
+from pathlib import Path
+
+import structlog
+
+from .config import Config
+
+log = structlog.get_logger()
+
+
+class SnapshotKeeper:
+    """Persistent ffmpeg per instance dumping latest frame to PNG file."""
+
+    def __init__(self, cfg: Config, snapshot_dir: Path) -> None:
+        self.cfg = cfg
+        self.snapshot_dir = snapshot_dir
+        self.snapshot_dir.mkdir(parents=True, exist_ok=True)
+        self._procs: dict[str, asyncio.subprocess.Process] = {}
+        self._tasks: dict[str, asyncio.Task] = {}
+        self._stop = False
+
+    def path(self, instance: str) -> Path | None:
+        """Возвращает Path к PNG если файл существует (хотя бы один кадр был
+        записан), иначе None — caller может fallback к cold ffmpeg."""
+        p = self.snapshot_dir / f"{instance}.png"
+        return p if p.exists() else None
+
+    async def start(self) -> None:
+        for inst in self.cfg.instances:
+            if not inst.output_rtsp_url:
+                continue
+            self._tasks[inst.name] = asyncio.create_task(
+                self._supervisor(inst.name, inst.output_rtsp_url))
+        log.info("snapshot_keeper.started",
+                 instances=list(self._tasks.keys()))
+
+    async def stop(self) -> None:
+        self._stop = True
+        for name, proc in self._procs.items():
+            try:
+                proc.terminate()
+                await asyncio.wait_for(proc.wait(), timeout=2)
+            except (ProcessLookupError, asyncio.TimeoutError):
+                try: proc.kill()
+                except ProcessLookupError: pass
+        for task in self._tasks.values():
+            task.cancel()
+        for task in self._tasks.values():
+            try: await task
+            except (asyncio.CancelledError, Exception): pass
+
+    async def _supervisor(self, name: str, url: str) -> None:
+        """Loop: launch ffmpeg, wait exit, retry с backoff."""
+        backoff = 3.0
+        while not self._stop:
+            try:
+                await self._launch_and_wait(name, url)
+            except Exception as e:
+                log.warning("snapshot_keeper.launch_fail",
+                            instance=name, error=str(e))
+            if self._stop:
+                break
+            await asyncio.sleep(backoff)
+            backoff = min(backoff * 1.5, 30.0)
+
+    async def _launch_and_wait(self, name: str, url: str) -> None:
+        out_path = self.snapshot_dir / f"{name}.png"
+        # fps=2 — 2 PNG/sec обновления (preview latency ~500 ms).
+        # -update 1 — overwrite single file атомарно (rename trick внутри
+        # image2 muxer).
+        proc = await asyncio.create_subprocess_exec(
+            "ffmpeg",
+            "-hide_banner", "-loglevel", "error",
+            "-rtsp_transport", "tcp",
+            "-i", url,
+            "-an",   # без аудио — не нужно для preview
+            "-vf", "fps=2",
+            "-update", "1",
+            "-f", "image2",
+            "-y", str(out_path),
+            stdin=asyncio.subprocess.DEVNULL,
+            stdout=asyncio.subprocess.DEVNULL,
+            stderr=asyncio.subprocess.PIPE,
+        )
+        self._procs[name] = proc
+        log.info("snapshot_keeper.launched", instance=name, pid=proc.pid)
+        rc = await proc.wait()
+        del self._procs[name]
+        # Read stderr (might give hint о cause)
+        try:
+            err = (await proc.stderr.read(2048)).decode(errors="replace") if proc.stderr else ""
+        except Exception:
+            err = ""
+        if not self._stop:
+            log.warning("snapshot_keeper.exited",
+                        instance=name, returncode=rc, stderr_tail=err[-300:])
diff --git a/controller/cuda_grid_controller/static/index.html b/controller/cuda_grid_controller/static/index.html
index 27a1596..6005d42 100644
--- a/controller/cuda_grid_controller/static/index.html
+++ b/controller/cuda_grid_controller/static/index.html
@@ -225,7 +225,10 @@ function initVideo() {
     busy = false;
   }
   tick();
-  setInterval(tick, 700);
+  // 250 ms = 4 fps preview. Keeper dumps PNG @ 2 fps в файл, /snapshot reads
+  // disk за ~4 ms — practical limit это keeper update rate (можно поднять
+  // до fps=4 в snapshot_keeper.py если нужно ещё быстрее).
+  setInterval(tick, 250);
   // expose для syncEditorBounds
   window.__previewEl = img;
   window.__previewSize = () => ({w: nativeW, h: nativeH});