Thanks guys, I actually contacted F5 support and managed to reduce the CPU usage significantly by doing the following:
1) reduced the length of the log entry by only logging the HTTP response and not the entire HTTP request
2) Since we also use a modified version of syslog-ng.conf to log things into different files, we've now remove all the regular expression from all the custom filters that we've created.
3) By modifying the syslog-ng.conf again, I was able to filter out all of our log messages from the alertd process which by default examine all message logged.
After the above 3 steps, the CPU 0 usage dropped from 100% to below 3% during load testing.