A little more information would help, is this a sudden event, when did the slowness began, any changes or installs in firewall?
What is slow, downloading/uploading files or just web browsing?
First conduct the regular systematic analisys prior to rebooting the firewall or clearing xlate table, have a look at this link for most commong points to check and start rulling out one by one.
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a008009491c.shtml
Rgds
Jorge
Jorge Rodriguez