Tiles requests have to go through GeoPrisma proxy. Since browsers limit concurrent requests based on hostname, this means that by configuring additional hostnames for the proxy, the browser will load more tiles simultaneously, resulting in a faster load time. The setup for GeoPrisma has to be made in common.php. Here's how it looks for this example:

org_geoprisma_SettingImpl::setProxyURLAliases(array( "http://geoprisma1.mapgears.com/samples/proxyurlaliases/proxy.php", "http://geoprisma2.mapgears.com/samples/proxyurlaliases/proxy.php", "http://geoprisma3.mapgears.com/samples/proxyurlaliases/proxy.php", "http://geoprisma4.mapgears.com/samples/proxyurlaliases/proxy.php" ));

If you inspect the tiles on the map, you'll see that they use the different hostnames set.

For this feature to work, your web server has to define those aliases and have to be added to your DNS as well. To see how it's done in Apache, see the ServerAlias property.