[Polizeiposaune]

Depending on implementation details of your CPU and OS, the syscall path may need to flush various auxillary caches (like one or more TLBs) to prevent speculation attacks, which may put additional "drag" on your program after syscall return.

[vlovich123]

Correct but you’d also still have that drag just from the kernel dirtying those caches in the first place.

But I was clarifying because the wording could be taken as data/instruction cache and there generally isn’t a full flush of that just to enter/leave kernel.

[slashdev]

Yes, that’s right. It seems to make up the bulk of the cost of a system call though, depending what it does, like read or write syscalls.