The transcode report on Special:Transcode_statistics (formerly Special:TimedMediaHandler) is really slow to load on Commons; I think it's doing at least a full table scan of the transcode table, which has over 5 million rows on Commons.
Additionally this report often cuts off the data and is hard to sort/read through. It needs some love and reworking to make it easier when managing transcode queue stuff!
Might need to add appropriate indexes to transcode table, might just need to make it read it more sensibly.
Change 979173 had a related patch set uploaded (by Brion VIBBER; author: Brion VIBBER):
[mediawiki/extensions/TimedMediaHandler@master] Adjust SQL queries on Special:TranscodeStatistics
https://gerrit.wikimedia.org/r/979173
Change 979173 merged by jenkins-bot:
The query tweaks didn't really help in practice. Time to add some sensible fixes. ;)
Change #1018347 had a related patch set uploaded (by Bvibber; author: Bvibber):
[mediawiki/extensions/TimedMediaHandler@master] WIP transcode table updates
https://gerrit.wikimedia.org/r/1018347