Bellow is a link to troubleshoot, it is just general to firewall itself but as in any other type of network troubleshooting identify what exactly is slow and when it became slow in order to know where to look. Usually but not always if it is three/four servers being slow when accessed from outside to inside and everything else is fine start at the bottom and work your way up through OSI by rulling out physical connections from the servers across to be clean, move on to uplink connections from switches to PIX/ASA, then PIX/ASA itself if ok then your router seating between ASA and ISP router, look into ISP router overall link and router performance etc.. if you can from outside internet conduct traceroute to servers and observe trace routes latency etc.. at least create a base line gathering all this information.. also this does not leave the questioned servers off the hook and again check their internal network connections and server overall performance as well.
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a008009491c.shtml
HTH
Rgds
Jorge
Jorge Rodriguez
