Why is Darnay’s journey slow?

France is in the grip of revolution. There is chaos, disorder, and confusion all around. The nation has been engulfed by a toxic wave of suspicion; the Revolutionary government is actively hunting down anyone who might conceivably represent a threat to the new regime. In such an environment, it's not surprising that the simple act of making a journey becomes a long, tedious, and dangerous process.
This is what happens to Charles Darnay. As well as the usual problem of bad roads, there's a noticeable shortage of fresh horses available. Even worse, travelers are being regularly stopped on the road by armed citizens who demand to see their identity papers. The situation is fraught with danger, especially for someone from an aristocratic family like Darnay.
Although he manages to proceed on his journey relatively safely for much of the way, things change dramatically for Darnay when he reaches the town of Beauvais. There he is denounced by the people as a traitor on account of his social background. Fortunately, he is saved from being torn to pieces by an irate, bloodthirsty mob thanks to the intervention of the postmaster. But Darnay's troubles are only just beginning, for he is subsequently taken to Paris to be imprisoned. Once again, the journey seems to take forever as the gates to the city are heavily guarded, and everyone's papers must be minutely examined before being allowed to enter.