When I bootup the developer pfsense, I notice there is a line that say "witness" is enabled and that this will slow down the system.
I did some digging on what it witness and it seems to be some sort of lock validation facility. With this turned on, it will degrade the system performance.
May I know is this only enabled in developer edition? Can turn it off?
Your running the developer edition. It is targeted to pfSense/FreeBSD developers who may want to debug an issue.
Compile a new kernel based on pfSense.6 if you want to get around it. Search your dev edition /home directories for it and copy it to /usr/src/sys/i386/conf/config/
See the FreeBSD handbook for more information related to kernel compiling.