18. A system for processing search queries, the system comprising: a plurality of query processing systems, each of which provides query-based access to a respective set of content, wherein the sets of content at least partially overlap with each other such that, for at least some search queries, different query processing systems are capable of providing the same content in response to the same search query; and a query optimization system comprising one or more computer processors, wherein the query optimization system generates, for each query processing system, performance data regarding an ability of the query processing system to respond to each of a plurality of classes of search queries, wherein the performance data is generated based on observed performance of the query processing systems in responding to search queries, wherein the query optimization system additionally responds to a search query from a user computing device by using at least the performance data generated for the plurality of query processing systems to select a particular query processing system to which to route the search query for processing, wherein the performance data includes query-class specific response data reflective of how accurately each query processing system can answer queries of a particular class, wherein the performance data indicates at least (1) an average response time associated with respective query processing systems in answering respective search queries and (2) recency of data stored by respective query processing systems, and wherein selection of the particular query processing system to route the search query for processing includes selecting query processing system having a system lowest average response time and/or selecting the particular query processing system storing more recent data.
20. The system of claim 18 , wherein the query optimization system responds to the search query from the user computing device by determining a class of the search query, and by using class-specific performance data associated with the class to select between the query processing systems.