You do, in fact. It’s called a memory write barrier. Ensures consistency of data structures as needed. And it call stall the cpu pipeline, so there’s a nontrivial cost involved.
No they don't. A fence only imposes ordering. It's instant. It can increase the chance of a stall when it forbids certain optimizations, but it won't cause a stall by itself.
CLWB is a small flush, but as tanelpoder explained the more recent CPUs did not need CLWB.