Feedback we received after DebConf17 was that the quality of the released videos was rather low. Since I'd been responsible for encoding the videos, I investigated that, and found that I had used the video bitrate defaults of ffmpeg, which are rather low. So, I read some more documentation, and came up with better settings for the encoding to be done properly.

While at it, I found that the reason that VP9 encoding takes forever, as I'd found before, has everything to do with the same low-bitrate defaults, and is not an inherent problem to VP9 encoding. In light of that, I ran a test encode of a video with VP8 as well as VP9 encoding, and found that it shaves off some of the file size for about the same quality, with little to no increase in encoder time.

Unfortunately, the initial VP9 encoder settings that I used produced files that would play in some media players and in some browsers, but not in all of them. A quick email to the WebM-discuss mailinglist got me a reply, explaining that since our cameras used YUV422 encoding, which VP9 supports but some media players do not, and that since VP8 doesn't support that format, the result was VP9 files with YUV422 encoding, and VP8 ones with YUV420. The fix was to add a single extra command-line parameter to force the pixel format to YUV420 and tell ffmpeg to also downsample the video to that.

After adding another few parameters so as to ensure that some metadata would be added to the files as well, I've now told the system to re-encode the whole corpus of DebConf17 videos in both VP8 as well as VP9. It would appear that the VP9 files are, on average, about 1/4th to 1/3rd smaller than the equivalent VP8 files, with no apparent loss in quality.

Since some people might like them for whatever reason, I've not dropped the old lower-quality VP8 files, but instead moved them to an "lq" directory. They are usually about half the size of the VP9 files, but the quality is pretty terrible.

TLDR: if you downloaded videos from the debconf video archive, you may want to check whether it has been re-encoded yet (which would be the case if the state on that page is "done"), and if so, download them anew.