"Using a syscall directly avoids that cost."

Quoting the author of the article,

"That said, this is nothing compared to the cost of a SYSTEM CALL. System calls are measured in usec, usually 10s of them, depending on the complexity of the system call you're calling. (In some cases, such as disk reads, it can even go to milliseconds.) I'd guess on average the cost of the transition is less than 1% of the system call you're making."