Spamassassin is slow - any tips or good commercial alternative?

[Charlie]

Hi,
I run an email server (Exim) which uses Mailscanner in combination with 
ClamAV and SpamAssassin. The server scans about 15,000 emails per day. 
SpamAssassin currently seems to take 5-10 seconds to scan even the smallest 
email, which is unacceptable for us.

I was wondering if anyone had either any tips on reducing the amount of time 
taken to scan an email for spam using SpamAssassin to 'well under one 
second', or if anyone can recommend a good, not too expensive, commercial 
alternative to SpamAssassin.

Thank you!
Charlie