With high safety, the statement does not complete until the update has been hardend in the log on the mirror. If you have a slow network connection, or the mirror has poor hardware, this can be painful with that many rows.

But that is still speculation, you need to gain knowledge what is actually taking time. On my web site there is a stored procedure, sp_sqltrace which permits you to run a batch and then compiles trace and wait-stats information about that batch. In this case the wait-stats information is the most important. If you see wait-stats with MIRROR in the name, the problem is as I suggested above.

You find sp_sqltrace on http://www.sommarskog.se/sqlutil/sqltrace.html.