The biggest CPU saving usually comes from avoiding unnecessary re-encoding. First check whether FFmpeg can copy the existing audio and video streams with -c copy; only decode, filter, and encode streams that actually need a change or do not meet the destination’s requirements. There is no reliable universal CPU-saving percentage: the result depends on your input, output, FFmpeg build, and hardware.
Find out whether FFmpeg is copying or re-encoding
FFmpeg’s documentation distinguishes streamcopy from transcoding: streamcopy copies streams without re-encoding, while transcoding decodes a stream and encodes it again. Encoding is computationally expensive, so streamcopy is the best first thing to test when your existing streams are already suitable for the destination. FFmpeg explains streamcopy and transcoding in its documentation.
A schematic example is:
ffmpeg -i INPUT -c copy OUTPUT
This is not a complete live-stream command. The right output muxer and destination URL, stream mappings, audio and video handling, and reconnect or process supervision depend on your setup. Test that the output plays correctly at the intended destination before relying on it continuously.
When streamcopy can work
Try streamcopy for the streams whose codecs and container are accepted by the destination, provided you do not need to alter them. If the destination requires a different codec or settings, or the output needs a filter such as scaling or an overlay, the affected stream may need to be processed and re-encoded.
#1 Best Overall
- Read Before You Buy — No Video Output: These adapters support charging and USB 2.0 data transfer, but cannot transmit video signals. Except for standard USB webcams (which use USB data only), they are not compatible with HDMI/DisplayPort cables, video-capable USB-C hubs, or docking stations with video output.
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When you need to transcode
Transcoding may be necessary when the output requirements differ from the input or a required transformation cannot be done by copying the encoded stream. Avoid treating it as an all-or-nothing choice: map streams deliberately, and copy any stream that needs no change while processing only the streams that require it.
Remove processing the ambient stream does not need
Review the command for filters and other transformations. Resizing, deinterlacing, overlays, audio resampling, and mixing are examples of work that can require processing. Remove unnecessary filters, but retain any changes needed for the intended picture or sound. Use deliberate stream selection and mapping so FFmpeg does not process streams the output does not need. FFmpeg’s documentation describes filtering and streamcopy.
Rank #2
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Do not confuse bitstream filters with re-encoding
A bitstream filter modifies encoded stream data without decoding it. It may suit a particular codec or container compatibility change, but it is not a general substitute for filters or transcoding. Use one only when the specific filter is appropriate and supported by both the input and output formats. See the FFmpeg bitstream-filter documentation.
-re is not a CPU-saving option
FFmpeg documents -re as reading file input at its native frame rate to simulate a capture device or live input. It does not turn encoding into streamcopy or, by itself, reduce the work required to encode. The manual cautions that a low read-rate limit on actual capture or live inputs can cause packet loss. Use it only when the input and delivery setup call for that behavior, not as a general CPU optimization. Check the FFmpeg manual for the option’s documented use.
Rank #3
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If encoding is required, tune for your actual target
Identify the encoder in use, then evaluate its settings against the output you need. For libx264, FFmpeg documents controls including preset and CRF. A preset trades encoding speed and resource use against compression efficiency and quality; there is no single preset that is best for every 24/7 ambient stream. Check the result against the required image quality, bitrate or bandwidth limit, latency, and destination compatibility. Confirm that your installed FFmpeg build includes the encoder and options you plan to use. FFmpeg’s all-in-one documentation covers codec options.
Do not assume hardware encoding is available or automatically preferable. Whether it is an option depends on the machine, hardware, FFmpeg build, and output requirements. Validate it on the system that will run the stream.
Rank #4
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Measure CPU use under the real workload
Compare streamcopy and transcoding only after checking that both outputs meet the destination’s requirements. For each valid option, observe the running FFmpeg process with the same input, filters, output settings, and target hardware. Also compare the output quality, bitrate, and latency. A result from another machine or stream does not establish the CPU use you will see.
A January 18, 2021 post on the FFmpeg-user mailing list describes an RTSP relay using video and audio streamcopy and reports low CPU use on the poster’s Raspberry Pi 3B+. It is an individual setup report, not a controlled benchmark or a result to expect on other systems. Read the mailing-list post.
Best Value
- 5-in-1 Connectivity: Equipped with a 4K HDMI port, a 5 Gbps USB-C data port, two 5 Gbps USB-A ports, and a USB C 100W PD-IN port. Note: The USB C 100W PD-IN port supports only charging and does not support data transfer devices such as headphones or speakers.
- Powerful Pass-Through Charging: Supports up to 85W pass-through charging so you can power up your laptop while you use the hub. Note: Pass-through charging requires a charger (not included). Note: To achieve full power for iPad, we recommend using a 45W wall charger.
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Match your guidance to the installed FFmpeg version
FFmpeg options and available encoders depend on the installed version and build. The project says its current documentation is regenerated nightly and advises consulting older-release documentation when relevant. Check the documentation for your version and confirm that your build supports the options you use. FFmpeg’s documentation portal provides version guidance.
Or let it run in the cloud
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Quick Recap
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