When enabling Federated Searching you have to be aware of the performance impact that this may have on the EV and SQL server.
Also, you can add more info to the webapp.ini file, below are some of the key elements - please note the webapp.ini file is case-sensitive and format aware (i.e. no spaces etc)
In order to control federated searching there are some new settings within WebApp.ini (located in WebApp.ini) being :-
FederatedSearchMaxVolSets This determines the maximum number of IndexVolumeSets to search simultaneously. Format :- FederatedSearchMaxVolSets=# where # is a number. Default :- 5 (if not set).
FederatedSearchMaxItems In order to control how IndexVolumeSet results are merged together there is a limit to the number of items that should be obtained from each IndexVolumeSet. That number is this property, MaxVolSetsItems. Consequently this number also determines the maximum number of results that a user would see matching his query. This is because it is possible that the most relevant results, after sorting etc. for his query came from ONLY one IndexVolumeSet. Format :-FederatedSearchMaxItems=# where # is a number. Default :- 1000 (if not set).
FederatedSearchTimeout This is a time limit to determine how long the federated search can take before it should be ended. Format :-FederatedSearchTimeout=# where # is a number of seconds. Default :-60 (if not set).
UseFederatedSearch This controls whether federated searching should be allowed for everyone who uses the WebApps. It maybe the administrator wants to turn off this functionality perhaps for configuration / performance reasons. Format :-UseFederatedSearch=# where # is a number. Default:-1 (if not set).