Season 4 update was supposed to nerf China’s Song villager production speed from 33% down to 25% (the patch notes claim it was from 35% down to 25% but I believe it was 33.33% before the update). The tooltip was never updated and most people including myself assumed it is just a visual problem. However, the actual production speed is still 33.33% which means a production time of 15 seconds.
I actually timed the production of villagers in an actual game. Producing 15 villagers took 225 seconds which is 15s per villager. Basically Song never got nerfed at the Season 4 update.
(one can probably also find the bug using the content editor)