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Proxy files can be generated with downsized resolutions. Is this the only reason why they work faster? In my experience, even lower resolution videos tend to lag before being replaced with their proxies.

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    $\begingroup$ I believe the encoding also plays a role. As far as I know, proxies are encoded as jpgs inside an avi container, which is presumably faster to decode (but much less space-efficient) than a "real" video codec. Though this is just speculation on my part $\endgroup$
    – gandalf3
    Nov 23, 2016 at 6:50
  • $\begingroup$ Yes that is correct, most modern codecs perform temporal compression (summing frames over time) whereas the jpg compression of the proxy workflow only compresses the current frame. This saves a LOT of processor time but sacrifices a LOT of HDD space $\endgroup$
    – 3pointedit
    Nov 30, 2016 at 22:56

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I think this is because of the codec the images are stored in. Compare replay speed from a video with replay speed from an image sequence. It doesn't go that fast, but at least faster than video. A video codec makes random access of certain frames that is needed for playback harder: if the compression uses data from precedent or following frames to reduce file size, Blender will need to load and decode a whole segment to read one frame. If some complicated math is used, it will slow down further.

When generating proxies, they not only have a lower resolution, but they are also in a codec that Blender can load at a decent speed (jpeg images or jpeg avi). The smaller file size allows for faster loading and keeping more data in RAM.

I think there are many reasons for proxy files being faster at playback, but to me, it's the random access to individual frames Blender gets that matters the most unless your video is 4k or your hard drive is at the other end of the world.

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