I tested the Farming 15% Speed Buff from the scroll obtained from Vocation badges and it doesn't work. I bought both the gathering and farming buff. I used the gathering one this morning and it does work. I used the farming one this afternoon and that one doesn't work. It's easy to tell because of the sound effect made from gathering/harvesting. The SFX for gathering under the buff was clearly cut off before the end but it wasn't for harvesting under the buff. I also compared the time it takes to harvest under the buff and it is the same as gathering without buff. Let me know if there's any details I should add.

P.S. I was harvesting barley. I didn't have another farming crop to test it.