Ideally, the computers running the primary and mirror servers should be configured with similar hardware (processor, disk, memory, and so on). At any given time, the database server running on either computer can be acting as the primary server for the database being mirrored. The mirror server utilization will typically be low, depending on update activity on the primary.
Query performance against the primary server is not affected by mirroring. The performance of transactions that update the database depends on the size of the transaction and the frequency of commits. A mirror server operating in asynchronous mode has better performance than one in synchronous mode, but is still slower than a database server that is not participating in a mirroring system. Performance is highly dependent on the speed of the network connection between the operational servers.