A client must seek all servers with a given preference number before searching for servers with the next higher number. It requires 5 or more seconds for a client to search for all the servers with a given preference number. This means that if you have a master server and 4 replicas in a domain, and you give each one a different preference number from 0 to 4, it could take a client more than 25 seconds to run through all of those preference levels.
To maximize performance, you should not use more than two or three levels of server preference. For example, in the case described above, it is better to give one of those five servers a preference=0 and all the others a preference of 1, or give two of them a preference of 1 and the remaining three a preference of 2.