Polonius loves to be the centre of attention and is not the "wisest" character in this play. While "performing" for the King, he allows the audience to become better acquainted with his character.
In line 97, "Therefore (since) brevity is the soul of wit..." Shakespeare cleverly tells us, through the words of Polonius, that in a clever speech, a few words convey the central meaning. However, Polonius is far from brief (and therefore we can presume far from clever) and continues to stumble on until he produces some love letters from Hamlet to Ophelia that he then shares with the King.
The tediousness of Polonius' performance wears even on Queen Gertrude as she states in line 101, "More matter with less art." Get on with it Polonius!
Note: the line numbers I have used are from the Folger Shakespeare Library version of the play.