  The observed delay of GRB 170817A relative to GW170817 carries significant
information about gamma-ray burst (GRB) physics and is subject to intense
debate. In this letter, we put forward an approach to discuss the major source
of this time delay. First of all, we use the structured jet model to fit the
X-ray/optical/radio afterglows of GRB 170817A together with superluminal motion
measured by the Very Long Baseline Interferometry. Our structured jet is
modelled with angle-dependent energy and baryon loading. It is found that our
model can well fit the afterglows of GRB 170817A. After that, the baryon
loading in the jet is inferred based on our fitting results. By comparing the
baryon loading to the mass outflow in different stages, we infer that the time
lag of the jet launch relative to the merger is less than hundreds or tens of
milliseconds. It suggests that the time delay of GRB 170817A relative to
GW170817 is defined mostly by the spreading time of the jet propagating to its
dissipation radius.
