[Exim] Spam Assassin.

Tried to set up spam assassin as a system-wide filter and my CPU went
through the roof. This is in a production environment and likely too many
connections are being made directly to server...unless somebody knows
something I don't (which is likely). I would like to use Spam Assassin to
filter based on .forward files for specific domains. I have all my users
segregated into different directories like so...

/usr/home/domain/spool
/usr/home/domain/spool
/usr/home/domain/spool

Suppose I wanted to set up a '.forward filter' for domain 2?
Can Spam assassin be set up to only filter using .forward and if so how?

It seems like this would be simple but I have really reached the end of my
rope. Please let me know if you require any more information or if I wasn't
very clear....