I've been asked in my job to violate the first normal form (repeating groups across columns, using empty / null values) several times, "for the sake of computer processing power". In a nutshell, a "student" table should have at least 8 empty fields (e.g telephones: telephone1, telephone2, telephone3...) instead of my suggestion - a "telephone" table which holds a telephone number (and possible other metadata) and the foreign key is the student id number. My boss says that it's better to store them that way because "there are less CPU cycles and that matters in web platforms", instead of using relations. I say that, in the worst of cases, it's negligible.
In that example, using relations (suppose that the tables are filled with lots of records in a medium-sized webapp) is notably slower than using that kind of table schema?