Userspace isn't slow, some kernel interfaces are
Title is click bait-y.  This has next to nothing to do with kernel interfaces and is all about network tuning and encapsulation.  Not sure why the authors went with the title as networking is interesting enough.Also, the "slow" things about kernel interfaces (if you aren't doing IO which is nearly always the slowest thing) usually isn't a given syscall, it's the transition from user to kernel space and back.   Lots of stuff going on such as flushing cache and buffers due to security concerns these days.