Speed is Valentine's clownish servant. He's got a quick wit and a habit of bagging on other characters in a way that reveals how foolish people can be. Check out what Speed says after Valentine brags about Silvia's beauty:
You never saw her since she was deformed.
How long hath she been deformed?
Ever since you loved her.
I have loved her ever since I saw her, and
still I see her beautiful.
If you love her, you cannot see her.
Because Love is blind. O, that you had mine eyes,
or your own eyes had the lights they were wont to
have when you chid at Sir Proteus for going
Obviously, Speed is pretty quick when it comes to clever dialogue. Instead of just coming out and saying to Valentine that love has blinded him, Speed engages his master in a dialogue that leads to him accusing Valentine of what amounts to wearing "love goggles." He also points out that it wasn't so long ago that Valentine was making fun of ("chid[ing]") Proteus for being in love with Julia.
Shakespeare's portrayal of Speed (and Lance) is relatively innovative. While servants in earlier sixteenth-century plays tended to mimic the behavior of their masters, Shakespeare does something new in Two Gentlemen. In the play, Speed's attitude toward marriage and love calls attention to his master's flaws. This is a pretty big deal because Shakespeare is one of the first playwrights to portray servants who are capable of defining the main characters. Compare Speed's role in the play to that of Lance, whose devotion to Crab draws our attention to Proteus's disloyalty to Julia and Valentine.
We also want to point out that Speed's character anticipates some of Shakespeare more infamous clown figures, like Feste in Twelfth Night. (Like Speed, Feste also has penchant for accurately pointing out the folly of those around him – he points out that Olivia's excessive mourning is over-indulgent and "foolish" and notes Duke Orsino's extreme moodiness when he compares Orsino's mind to an "opal" that changes color.)