If you return fewer columns there is less data to go across the network and less data for the database to process and it will almost always return faster. Databases also tend to be slower using select * because the database then has to go figure out what the columns are and thus do more work than when you specify. Further select * will often return bad results if the structure changes significantly. It may end up showing the user fields you don;t want them to see or if someone is silly enough to rearrange the columns, then the application may actually appear to show things in the wrong order or if doing an insert from the data, put them in the wrong column. It is almost alawys a poor practice to use selct * in production code.
