Why Plex video preview thumbnails take so long, and how to speed them up

Plex makes video preview thumbnails inside Plex Media Server, on the CPU. Plex's support articles say the job "essentially requires a transcode" of each file, and call it CPU-intensive. By default Plex grabs a frame every 2 seconds, and it runs the job during its scheduled maintenance. So a large library can take "hours or even days". To speed it up:

• Generate only for the libraries that need it.
• Leave key-frame-only extraction on.
• Raise the frame interval.
• Or move the work to a GPU with an external tool such as Media Preview Generator.

Why it is slow

• It decodes the video on the CPU. Plex describes the job as CPU-intensive, taking "anywhere from less than a minute to several minutes for a single item". The time depends on the processor and on the file's length and resolution (Plex Support). Plex documents no GPU option for this job.
• It samples often. Plex's documented default is one thumbnail every 2 seconds (GenerateBIFFrameInterval, Plex Support).
• It runs on a schedule. You choose "never", "as a scheduled task", or "as a scheduled task and when media is added" (Plex Support). Plex warns that turning it on before a large import can keep the server busy "for hours or even days". Users report it working through one file at a time during the maintenance window (forum reports, not a Plex statement).
• It shares the machine with playback. The same CPU is serving your streams.

Speed it up with Plex's own settings

Try these first. They cost nothing.

1. Limit it to the libraries that need it. Preview generation can be turned off per library. It is an advanced setting when you create or edit a library (Plex Support). Home videos or rarely watched libraries may not need previews at all.
2. Keep key-frame-only extraction on. GenerateBIFKeyframesOnly "greatly speeds up processing and reduces CPU usage". It defaults to on. Check that nobody turned it off.
3. Raise the frame interval. GenerateBIFFrameInterval sets the seconds between thumbnails. Going from 2 to 5 or 10 means far fewer frames, with coarser scrubbing. Both are hidden settings: they live in Preferences.xml on Linux, the registry on Windows, and a .plist on macOS. Restart Plex after changing them. Plex documents both in Advanced, Hidden Server Settings, last modified 8 December 2022. We have not confirmed they behave the same on every current Plex release.
4. Don't turn on "when media is added" right before a big import. Plex's own warning: the server may spend hours or days on it.

If that is fast enough, stop here. The built-in needs no extra software.

Move the work to a GPU

Media Preview Generator is a free, MIT-licensed Docker app. It makes the same BIF files Plex would, using FFmpeg with GPU decoding (NVIDIA, Intel or AMD). It then writes them into Plex's data folder, where Plex serves them to its apps. A full scan can run for hours while new imports, set to High priority, jump ahead of it. It differs from the built-in in several ways:

• It uses the GPU. Decoding and downscaling happen on the GPU. If a file won't decode there, another GPU tries it, then the CPU (CPU fallback).
• It works on several files at once. You set up worker groups: a group holds the GPUs and CPU you choose, each with its own worker count and allowed jobs, and the group has its hours.
• It works per file, when the file arrives. Triggers include Sonarr and Radarr webhooks, Plex's own webhook (Plex Pass), a "Recently Added" poll (no Plex Pass), schedules, or a manual pick. See Previews as soon as Sonarr or Radarr imports a file.
• It can use another machine's GPU. Add that machine as a worker node, or run the app there with network access to Plex and the Plex data folder mounted read-write.

Once it is running, set Plex's Settings → Library → Generate video preview thumbnails to Never, so Plex doesn't do the same work again. Setup steps are in Generate Plex preview thumbnails with a GPU.

Limits to know first

• It runs only in Docker. It has a web UI and no CLI.
• Plex must scan a new file before its preview can be written, because the file's location in Plex's data folder only exists after the scan. The app retries automatically after 1, 2, 5, 15 and 60 minutes by default. See Slow-backoff retry queue.
• Optional chapter thumbnails use CPU, are off by default, and currently support Plex 1.43.4.x or 1.43.5.x; they are separate from GPU-accelerated scrubber previews.
• On Windows only NVIDIA GPUs are accelerated, and on macOS none are. Those setups run on CPU.

Related

• Comparison with the built-in generators
• Getting Started
• HDR and Dolby Vision thumbnails: if Plex's HDR previews look washed out or green