The system is now pre-resolving hostnames to IP addresses and where possible using the IP address directly.
For HTTP and MySQL tests using the IP address directly is not possible (because in HTTP the hostname forms part of the request to the server and in MySQL it often determines where the user is seen as coming from, for example a user allowed from 'localhost' could not connect to the host IP 127.0.0.1 directly as user@127.0.0.1 is not in the permissions table). For these tests the hostname is stripped out and resolved using gethostbyname(). This hopefully leaves it cached and so doesn't cause any DNS delays.