I am learning Potionmaking II - Fast! Fast! I imbibed an Essence of Yellow Crumb Flower and my speed of learning did not appear to change at all. I suspect that if you are already learning as fast as you can, then the Yellow Crumb essence has no effect. If this is not a bug, then it would be helpful to note so in the description so that Glitches do not waste this precious (and very expensive) commodity.