Agreed, I think they forgot to scale up the specialists after buffing all the other tile yields in the game significantly (those 6 food farms would be almost gamebreakingly strong in civ5, or even civ4).
They are also double nerfed, because apart from being out-produced by pretty much every other tile, they also no longer provide GP-points, which was the main reason you worked those slots in 4 and 5 anyways.
The slots are pretty much only worth working if all other tiles are worked, or if you are at the housing cap which makes food-tiles useless.