In current practice, the common approach to perform server selection is to leave all responsibility to the user. Hence, server selection should be automated, because an automated process can take advantage of information that is either not available or without meaning to the typical end user. Server processing time is heavily dependent on the server's configuration. However, the impact of a web server's performance and observed load on overall response time is small.
Parallel retrieval has a considerable potential of reducing response time.
On the other hand, parallel retrieval also potentially impairs other users' observed network performance. Examples can be given, where the optimal server varies for differing document sizes. Hence, ideally, server selection should take document sizes into account, because there is a potential profit. A more realistic algorithm, on the other hand, strongly relies on heuristics and approximations and needs to be tailored for and evaluated in a specific environment or application.