Say I could use either of these two queries:
SELECT name, age FROM people WHERE person_id = '1';
SELECT name, age, address, email FROM people WHERE person_id = '1';
Would the first one be significantly faster?
I doubt there would be a measurable difference. Maybe if one of the additional fields in the second query contained large amounts of data, there would be some additional time needed on each row fetch to copy that data from the DB to the application memory -- but if you need that data, then you'll have to use that time anyway -- and if you don't need that data, then why would you want it in the query results in the first place?