Are there differences to drywall joint compound and plaster?
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.