Vehicular ad hoc networks aim at increasing passenger safety by exchanging warning messages between vehicles wirelessly. A main challenge is to resist to various malicious abuses and security attacks. However, any security mechanism comes with overhead. We analyze how the authentication algorithm ECDSA and the consensus mechanism impact the vehicular network performance and the braking distance. Processing and communication overheads, decision methods for consensus, are analyzed by analytical models and intensive simulations. We propose a formula to assess the total time overhead of the authentication. Results conclude that the authentication key size should be chosen carefully, and the decision method should be adapted to the context.