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On one nearly static vertical video, FFmpeg’s libx264 -preset slow produced a file just 3,398 bytes smaller than medium—a 0.16% reduction. On a separate synthetic noisy clip, the reduction was 2.17%, but encoding took much longer. The result is not a universal preset ranking: content changed the trade-off, and the measurements came from one two-core ARM server.
What the 0.16% result actually means
In an article published September 15, 2026, Obole compared FFmpeg 6.1.1-3ubuntu5 using libx264 on a two-core ARM Neoverse-N1 server with 11 GiB of RAM and no GPU. The 0.16% figure is the measured file-size reduction from slow versus medium on a 61.80-second, 1080×1920, 30 fps clip that was almost motionless: white text against black. The medium output was 2,074,372 bytes; slow saved 3,398 bytes. Obole’s test and results.
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That small difference is specific to this source, encoder, and machine. It does not show that slow is pointless in general. On a second 12-second, 1080×1920, 30 fps source—a deliberately difficult synthetic pattern with temporal noise—slow saved 250,481 bytes, or 2.17%, against an 11,538,029-byte medium output. That source is synthetic, not representative phone or camera footage.
How the ten settings were compared
The test varied x264 presets and CRF values, as well as a two-pass bitrate-targeted encode. Obole recorded exact output size, elapsed wall-clock time, SSIM, and PSNR. Most settings were repeated four times; intermediate CRFs were repeated twice. File size and quality scores were consistent across repeats, but run time varied. The synthetic source was already an H.264 encode, so its reference was compressed before these comparisons.
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The server ran the settings serially. The reported measurements therefore describe that particular workload on that particular two-core ARM system, not a controlled comparison across processor types or hardware encoders. The test excluded x86 processors, GPUs, other codecs, real phone footage, and subjective viewing evaluations.
Why matching CRF numbers can mislead
CRF is a quality-target mode in which output size can vary; it is not a promise that two different presets at the same CRF will produce equivalent quality or size. A preset changes how much encoding search effort x264 spends. FFmpeg’s libx264 documentation describes the crf quality option and preset encoding option.
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On the near-static clip, veryfast at CRF 23 produced 1,873,445 bytes with luma SSIM 0.999581. medium at CRF 23 produced 2,074,372 bytes with luma SSIM 0.999741. Those are not same-size or same-quality comparisons. To make a more useful size comparison, Obole also considered medium at CRF 25 and 26 against veryfast at CRF 23. In that near-static test, veryfast was reported to encode 24–32% faster than medium at similar size.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThe synthetic clip yielded a different result. medium at CRF 24 made a 9,175,052-byte file with SSIM 0.915240; veryfast at CRF 23 made a 10,709,432-byte file with SSIM 0.914306. In this single test, medium was smaller and had a slightly higher SSIM, at about twice the encoding time. The contrast is a reminder that a preset’s practical value depends on the material being encoded.
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What quality metrics can—and cannot—tell you
SSIM and PSNR are objective comparisons against a reference, not direct measurements of whether a person will notice a difference. Obole’s FFmpeg build did not have VMAF available, and the article reports no blind viewing test. Treat the scores as evidence for comparisons within these test conditions, not as a guarantee about perceived quality in every scene or on every screen.
For the nearly static clip, the encoded video is only part of the file. Its AAC audio track at 128 kb/s measured 875,130 bytes—42% of the CRF 23 output and 55% of the CRF 32 output. As a result, a reduction in video-stream size does not translate to the same percentage reduction in the complete MP4.
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CRF, bitrate targeting, and the two-pass result
With CRF, you choose a quality target and let the video bitrate and size vary with the content. With -b:v, you specify a target video bitrate and accept that visual quality can vary as the encoder fits the material to that rate. Two-pass encoding analyzes the video in one pass and uses that information in the second pass to meet a bitrate target more evenly.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallIn Obole’s two-pass 2 Mb/s test, the output was 5,839,424 bytes from the 2,880,255-byte near-static source. On the synthetic clip, a medium CRF 32 encode was smaller at a similar reported SSIM. Those outcomes describe the chosen inputs and target; they do not establish that bitrate targeting is always a poor choice. It can still make sense when a delivery platform or storage budget requires a particular bitrate or approximate size.
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A practical starting command for a simple social video
For the author’s own simple video, Obole used this command:
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset veryfast
-pix_fmt yuv420p -c:a aac -b:a 128k -movflags +faststart output.mp4On the author’s material and test server, this produced a 1,873,445-byte file in 21.77–22.82 seconds. That timing and result belong to the stated test setup; they are a reproducible starting point, not an established optimum for other footage, machines, or delivery requirements.
How to choose a preset for your own footage
- Pick representative clips. Include the kinds of scenes your videos actually contain: static titles, movement, detail, noise, and dark or textured areas where relevant. A text-on-black clip alone will not predict results on complex footage.
- Hold the conditions steady. Use the same input, resolution, frame rate, audio settings, and output container when comparing presets. Run the encodes on the machine you plan to use.
- Compare at similar output size, not just identical CRF. Adjust CRF across presets until outputs are roughly comparable in size, then examine quality and elapsed time. Matching CRF values alone does not make the quality comparison fair.
- Inspect the video as well as the numbers. Compare the same scenes side by side, especially motion and fine detail. SSIM or PSNR can support the comparison, but neither replaces viewing.
- Choose for the constraint that matters. If encoding time is costly and the visual result is acceptable, a faster preset may be preferable. If smaller output matters more and the slower encode earns a worthwhile reduction on your material, use the slower option. Recheck when the source type or machine changes.
Obole’s correction note says an earlier claim of a 43% picture-size reduction between CRF 23 and CRF 32 was recalculated to 41.1% from the published byte counts. It also clarifies that the reported slow gains use medium as the comparison baseline for both sources.
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