There are huge differences. I am sure a professional tradesman can go over the exact specifics but here is my common man take (even though I have drywalled too many houses to remember):
- plaster generally sets more quickly
- plaster is thicker
- plaster can be applied more thickly (joint compound you get about 1/8")
- plaster to me is like working with thinset except you will see the plaster as thinset is almost always hidden
- joint compound can be sanded more easily
- joint compound is more prone to cracking (this is about application too)
- I would personally only use plaster for a very small area that is having cracking problems. I have done some plaster work and literally drove me nuts. Also I do not consider myself artistic. When applying plaster it is an art. If you over apply sanding it down is way harder than joint compound. I would never think about plastering all of a drywall in a house.
The only thing else I can add is I don't know what they consider slow plaster. I am sure that refers to the setting time. The plaster I have used starts setting in 30-60 minutes (hence my reference to thinset). Maybe they have something that lasts longer or maybe it is some combo plaster/compound.