Depends on a lot of conditions, allready how much your PIX is occupied yet and then on your logging level, if you using "warnings" then this should not have such a great performance impact. If you use informational then you will get a lot of informations that will impact more the performance.
Command Reference:
http://www.cisco.com/en/US/products/sw/secursw/ps2120/products_command_reference_chapter09186a00801727a9.html#wp1028090
Specify the syslog message level as a number or string. The level you specify means that you want that level and those less than the level. For example, if level is 3, syslog displays 0, 1, 2, and 3 messages. Possible number and string level values are:
•0—emergencies—System unusable messages
•1—alerts—Take immediate action
•2—critical—Critical condition
•3—errors—Error message
•4—warnings—Warning message
•5—notifications—Normal but significant condition
•6—informational—Information message
•7—debugging—Debug messages and log FTP commands and WWW URLs
sincerely
Patrick
