We currently look up the peer hostname in order to perform certificate verification while accepting SSL (TLS) connections. This blocks the event loop thread in cases where it has to go over the network. We saw one issue where a misconfiguration meant that this would block for 15 seconds.
Once we add asynchronous DNS lookup facilities (MESOS-9338), we can use them to avoid blocking the event loop thread.
We should consider logging slow DNS reverse lookups and adding timing metrics for them.