IVs, EVs, Nature, Stat Boosts, Held Items, or anything else that might effect it.
Speed Swap exchanges the Speed stats of the user and the target. The two Pokémon's Speed stat stages are ignored and are not exchanged.
IVs, EVs, and natures are affected. Speed swap also doesn't affect held items like iron ball and choice scarf.
Source: Bulbapedia
