It goes by unit, you shouldnt have to do each individual. I prefer to walk them over to the storehouse and then click feed, speeds things up a bit. Peasants do have to walk out to them and hand them food, they do not go to the Inn as other workers do. So if you are a bit short on workers it could slow down a lot depending on unit size, etc.

As this is your first time with K&M, you should get used to it now. Things take a lot of real time in general. Feeding troops is one of the longer functions. Wait until you try and get leather armor working... :) All worth it though, a great game.