If there is a need to do a comparative study between performance of the database server in Synchronous Mirror and while in Asynchronous Mirror, the SQLServer counter Transaction delay should be captured and analysed.
Transaction Delay: Delay (in milliseconds) in waiting for commit acknowledgement from the mirror. This counters reports the total delay for all the transactions in process at that time.
When running in Asynchronous mirroring the Transaction Delay counter will always be 0.
Find Transaction delay in Synchronous Mirror