If you go to Business Contact Manager -> Manage E-mail Auto-linking,
you'll have an option to link all the emails of all your contacts.
From there, you can either click OK to have all future emails be
linked to their contacts, or you can click Search and Link to link all
of your existing emails (which is a time-consuming and processor-heavy
task, but worth it, so you should probably do it overnight or over the
weekend).  After that, every time you make a new contact, you have to
manually select them to be auto-linked, which you can do from the
contact window.

My suggestion would be to try it, and if it does affect performance,
then turn it off.  I know that our company hasn't experienced any
problems with email auto-linking, and it's a very convenient feature
to have.