Lead-free halide double perovskites A2MM'X6 (A = Rb+, Cs+, etc.; M = Ag+, K+, Li+; M' = Sb3+, In3+ or Bi3+ and X = I-, Br- or Cl-) have recently been suggested as an alternative to lead-based halide perovskites for optoelectronic and photovoltaic applications. While a great deal of effort has been put into device engineering to improve the performance of photovoltaic and optoelectronic devices that are based on A2MM'X6 double perovskites, there has been relatively little attention given to their inherent photophysical properties. The current research demonstrates that small polaron formation under photoexcitation and polaron localization limit the carrier dynamics in the Cs2CuSbCl6 double halide perovskite. In addition, temperature-dependent AC conductivity measurements indicate that single polaron hopping is the primary conduction mechanism. The ultrafast transient absorption spectroscopy results revealed that the formation of small polarons, which function as self-trapped states (STSs) and result in the ultrafast trapping of charge carriers, are caused by a distorted lattice under photoexcitation. These findings provide a thorough understanding of the intrinsic restrictions of Cs2CuSbCl6 perovskite and could have relevance for other antimony-based semiconductors.