What's the fastest way to remove/dissolve plaster from brick?
Sandblasting would be the fastest, but it's not a easy DIY project.

Muratic acid, properly used, vented, masked (organic cartridges), gloved, goggled.  A 10:1 water:acid mix is a good place to start.  Be sure to add acid to water during dilution.   

You should hand scrape or carefully grind with a diamond cup wheel to leave a thin a layer as possible (&lt; 1/16 inch).

Let soak 10-20 min and scrub with acid safe brush.  

Once clean, several water only rinses, then use an acid neutralizer.
Baking soda and water, diluted lime or diluted ammonia will work.  Final water rinse.

If any walls are in kitchens, a natural stone/grout sealer will greatly ease future cleaning.  You may also like the look of an enhancing sealer, but that is a personal preference if mine.