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I have found this solution provided by hackerb9:

ffmpeg  -i input.mp4  -b:v 0  -crf 30  -pass 1  -an -f webm -y /dev/null
ffmpeg  -i input.mp4  -b:v 0  -crf 30  -pass 2  output.webm

It's works, but the second pass require more than 10 hours (for 30 minutes of video HD).

Now the question is: can I do the same things in a reasonable time?

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To speed it up you need to increase the compute power and throughput of your computer. I think on my machine the I/O is the bottleneck, but yours may be different so look at upgrading any or all of the following:

  • CPU - a faster processor allows for more operations per second
  • Hard drive - running all video operations off a separate drive to your operating system is essential for speed. For even higher throughput, you can use a RAID array of drives
  • Entire system - running multiple computers in parallel can, well, parallelise lengthy operations like this. This may be overkill for what you want

It may be simplest for you to just buy a new machine.

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  • Thank you, I had considered several of these options. I have done the conversion by shutting down all other tasks and the Wi-Fi network. Regarding the 'Ferrari', I would say that it's all too easy an answer. :-) May 23 at 14:17
  • I used to do work with cgi rendering, and the only way to do it in a sensible timeframe was to have multiple machines each with multiple processors, using a high speed RAID array. Video is not quite as computationally expensive, but it is a simple solution to get a better machine, if you have the budget
    – Dr Mayhem
    May 23 at 15:46

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