OP: Please don't take this the wrong way, but given some of the statements and questions in your post I fear that you would not be able to either a) understand some of the output/recommendations monitoring would do, b) could do monitoring improperly which could make performance suffer even more, c) would not know how to fix findings or d) possibly (and worst of all by far) make some changes that could be disastrously bad. A non-functional production system is NOT the place to learn by trial and error!

You mentioned Enterprise Manager. Was that just a slip or are you really on a VERY old edition of SQL Server? 🙂

I agree with Grant that encryption of mirroring would be an unlikely cause of what you are seeing.

Note that having mirroring in play can introduce a number of performance issues depending on many factors.