I’m sorry, but you people can’t read. 
It also happens with starch(from cattails): 40 minutes, 48 minutes and 1h12 (72 minutes).
It functions more as expected (but still “wrong” for >3) with protein powder:
- 9 minutes total for batch of 1
- 9 minutes total for batch of 2
- 9 minutes total for batch of 3
- 9 minutes 24 seconds total for batch of 4
Honestly, if IRL I had two friends to assist then for an item that takes X time I’d expect that crafting a batch of 1 would take X/3, batch of two 2X/3, batch of 3 3X/3 = X for items that are bottle-necked by labour (like starch), or X, X, X, 2X, 2X, 2X, 3X, … for items that are bottle-necked by time (e.g. cooking). And of course X, 2X, 3X, 4X for items that are bottle-necked by tools (cooking something and only having 1 container for cooking). The actual time in-game fits none of this.
Looking back at the starch example, if I have 2 NPCs, then I have 3X labour points to spend. It takes X = 40 minutes for a batch of 1 starch from cattail rhizomes (see JSON) and it must take 120 labour points for all of them. However, it takes:
- 1: 40 points
- 2: 2*48 = 96 points (16 over expected)
- 3: 3*72 = 216 points (96 over expected)
- etc