Sonnet 51: "Thus can my love excuse the slow offence..."

Thus can my love excuse the slow offence,
 Of my dull bearer, when from thee I speed,
 From where thou art, why should I haste me thence?
 Till I return of posting is no need.
 O what excuse will my poor beast then find,
 When swift extremity can seem but slow?
 Then should I spur though mounted on the wind,
 In winged speed no motion shall I know,
 Then can no horse with my desire keep pace,
 Therefore desire (of perfect'st love being made)
 Shall neigh (no dull flesh) in his fiery race,
 But love, for love, thus shall excuse my jade,
 Since from thee going, he went wilful-slow,
 Towards thee I'll run, and give him leave to go.