On my laptop, syscall(SYS_getpid) takes 241.65 ns if syscall auditing is on (even with no rules) and and 70-ish ns if syscall auditing is off.

On my desktop, getpid takes ~65.5 ns on a default Fedora configuration (I think -- this system has been through a few upgrades), but if I do auditctl -a task,never (which is a hack that Oleg Nesterov rigged up fairly recently), getpid goes down to 40.37 ns.

I think that, for users who don't have any manually configured audit rules, running every system call through the system call slow path is a poor tradeoff.  Please consider changing the default rules (in /etc/audit/audit.rules or wherever the best place is) to '-a task,never'.  Users who configure syscall auditing for real can easily remove that rule and replace it with whatever they want.