Are Americans inefficient? A few hours later, my foreign teacher in Chicago sent him a short message, sometimes the next day, just yesterday afternoon. ...

Some people are very low. He may just be a freeloader, or he doesn't pay enough attention to this job. However, some people may not be used to short messages, so email will be better. Anyway, some teachers in America send him emails or something, and his reply will be slow. Some may be soon.